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Copyright,  1945,  by  H.  D.  Smyth 

Reproduction  in  whole  or  in  part 
authorized  and  permitted 

Printed  in  the  United  States  of  America 
by  Maple  Press,  York,  Pennsylvania 


THE  story  of  the  development  of  the  atomic  bomb  by  the 
combined  efforts  of  many  groups  in  the  United  States  is  a 
fascinating  but  highly  technical  account  of  an  enormous  enter- 
prise. Obviously  military  security  prevents  this  story  from  being 
told  in  full  at  this  time.  However,  there  is  no  reason  why  the 
administrative  history  of  the  Atomic  Bomb  Project  and  the  basic 
scientific  knowledge  on  which  the  several  developments  were 
based  should  not  be  available  now  to  the  general  public.  To  this 
end  this  account  by  Professor  H.  D.  Smyth  is  presented. 

All  pertinent  scientific  information  which  can  be  released  to 
the  public  at  this  time  without  violating  the  needs  of  national 
security  is  contained  in  this  volume.  No  requests  for  additional 
information  should  be  made  to  private  persons  or  organizations 
associated  directly  or  indirectly  with  the  project.  Persons  dis- 
closing or  securing  additional  information  by  any  means  whatso- 
ever without  authorization  are  subject  to  severe  penalties  under 
the  Espionage  Act. 

The  success  of  the  development  is  due  to  the  many  thousands 
of  scientists,  engineers,  workmen  and  administrators — both 
civilian  and  military — whose  prolonged  labor,  silent  perseverance, 
and  whole-hearted  cooperation  have  made  possible  the  un- 
precedented technical  accomplishments  here  described. 


Major  General,  USA 
War  Department 
Washington,  D.  C. 
August  1945 


THE  ultimate  responsibility  for  our  nation's  policy  rests  on 
its  citizens  and  they  can  discharge  such  responsibilities  wisely 
only  if  they  are  informed.  The  average  citizen  cannot  be  expected 
to  understand  clearly  how  an  atomic  bomb  is  constructed  or  how 
it  works  but  there  is  in  this  country  a  substantial  group  of  engi- 
neers and  scientific  men  who  can  understand  such  things  and 
who  can  explain  the  potentialities  of  atomic  bombs  to  their  fellow 
citizens.  The  present  report  is  written  for  this  professional  group 
and  is  a  matter-of-fact,  general  account  of  work  in  the  United 
States  since  1939  aimed  at  the  production  of  such  bombs.  It  is 
neither  a  documented  official  history  nor  a  technical  treatise  for 
experts.  Secrecy  requirements  have  affected  both  the  detailed 
content  and  general  emphasis  so  that  many  interesting  develop- 
ments have  been  omitted. 

References  to  British  and  Canadian  work  are  not  intended  to 
be  complete  since  this  is  written  from  the  point  of  view  of  the 
activities  in  this  country. 

The  writer  hopes  that  this  account  is  substantially  accurate, 
thanks  to  cooperation  from  all  groups  in  the  project;  he  takes 
full  responsibility  for  such  errors  as  may  occur. 

H.  D.  SMYTH 

July  /,  1945 

Minor  changes  have  been  made  for  this  edition.  These  changes 
consist  of  the  following  variations  from  the  report  as  issued 
August  12,  1945:  (1)  Minor  clarifications  and  corrections  in 
wording;  (2)  Inclusion  of  a  paragraph  on  radioactive  effects 
issued  by  the  War  Department  to  accompany  the  original  release 

viii  Preface 

of  this  report;  (3)  Addition  of  a  few  sentences  on  the  success  of 
the  health  precautions;  (4)  Addition  of  a  few  names;  (5)  Addition 
of  Appendix  6,  giving  the  War  Department  release  on  the  New 
Mexico  test  of  July  16,  1945;  (6)  Inclusion  of  the  photographic 
section;  (7)  Inclusion  of  an  index. 

H.  D.  S. 

September  7,  1945 







DECEMBER  1941  45 



CHICAGO  IN  1942  88 


AS  OF  FEBRUARY  1943  108 


TO  JUNE  1945  130 














1.1.  The  purpose  of  this  report  is  to  describe  the  scientific  and 
technical  developments  in  this  country  since  1940  directed  toward 
the  military  use  of  energy  from  atomic  nuclei.  Although  not 
written  as  a  "popular"  account  of  the  subject,  this  report  is 
intended  to  be  intelligible  to  scientists  and  engineers  generally 
and  to  other  college  graduates  with  a  good  grounding  in  physics 
and  chemistry.  The  equivalence  of  mass  and  energy  is  chosen  as 
the  guiding  principle   in   the  presentation  of  the  background 
material  of  the  "Introduction." 


1.2.  There  are  two  principles  that  have  been  cornerstones  of 
the  structure  of  modern  science.  The  first — that  matter  can  be 
neither  created  nor  destroyed  but  only  altered  in  form — was 
enunciated  in  the  eighteenth  century  and  is  familiar  to  every 
student  of  chemistry;  it  has  led  to  the  principle  known  as  the  law 
of  conservation  of  mass.  The  second — that  energy  can  be  neither 
created  nor  destroyed  but  only  altered  in  form — emerged  in  the 
nineteenth  century  and  has  ever  since  been  the  plague  of  in- 
ventors of  perpetual-motion  machines;  it  is  known  as  the  law  of 
conservation  of  energy. 

1.3.  These  two  principles  have  constantly  guided  and  disci- 
plined  the   development   and   application   of  science.    For   all 
practical  purposes  they  were  unaltered  and  separate  until  some 
five  years  ago.  For  most  practical  purposes  they  still  are  so,  but 
it  is  now  known  that  they  are,  in  fact,  two  phases  of  a  single 
principle  for  we  have  discovered  that  energy  may  sometimes  be 
converted  into  matter  and  matter  into  energy.  Specifically,  such 
a  conversion  is  observed  in  the  phenomenon  of  nuclear  fission  of 


2  Introduction 

uranium,  a  process  in  which  atomic  nuclei  split  into  fragments 
with  the  release  of  an  enormous  amount  of  energy.  The  military 
use  of  this  energy  has  been  the  object  of  the  research  and  pro- 
duction projects  described  in  this  report. 


1.4.  One  conclusion  that  appeared  rather  early  in  the  develop- 
ment of  the  theory  of  relativity  was  that  the  inertial  mass  of  a 
moving  body  increased  as  its  speed  increased.  This  implied  an 
equivalence  between  an  increase  in  energy  of  motion  of  a  body, 
that  is,  its  kinetic  energy,  and  an  increase  in  its  mass.  To  most 
practical  physicists  and  engineers  this  appeared  a  mathematical 
fiction  of  no  practical  importance.  Even  Einstein  could  hardly 
have  foreseen  the  present  applications,  but  as  early  as  1905  he 
did  clearly  state  that  mass  and  energy  were  equivalent  and  sug- 
gested that  proof  of  this  equivalence  might  be  found  by  the  study 
of  radioactive  substances.   He  concluded  that  the  amount  of 
energy,  E,  equivalent  to  a  mass,  m,  was  given  by  the  equation 

E  =  mc2 

where  c  is  the  velocity  of  light.  If  this  is  stated  in  actual  numbers, 
its  startling  character  is  apparent.  It  shows  that  one  kilogram 
(2.2  pounds)  of  matter,  if  converted  entirely  into  energy,  would 
give  25  billion  kilowatt  hours  of  energy.  This  is  equal  to  the 
energy  that  would  be  generated  by  the  total  electric  power 
industry  in  the  United  States  (as  of  1939)  running  for  approxi- 
mately two  months.  Compare  this  fantastic  figure  with  the  8.5 
kilowatt  hours  of  heat  energy  which  may  be  produced  by  burning 
an  equal  amount  of  coal. 

1.5.  The  extreme  size  of  this  conversion  figure  was  interesting 
in  several  respects.  In  the  first  place,  it  explained  why  the  equiva- 
lence of  mass  and  energy  was  never  observed  in  ordinary  chemical 
combustion.  We  now  believe  that  the  heat  given  off  in  such  a 
combustion  has  mass  associated  with  it,  but  this  mass  is  so  small 
that  it  cannot  be  detected  by  the  most  sensitive  balances  avail- 

Introduction  3 

able.  (It  is  of  the  order  of  a  few  billionths  of  a  gram  per  mole.) 
In  the  second  place,  it  was  made  clear  that  no  appreciable 
quantities  of  matter  were  being  converted  into  energy  in  any 
familiar  terrestrial  processes,  since  no  such  large  sources  of  energy 
were  known.  Further,  the  possibility  of  initiating  or  controlling 
such  a  conversion  in  any  practical  way  seemed  very  remote. 
Finally,  the  very  size  of  the  conversion  factor  opened  a  magnifi- 
cent field  of  speculation  to  philosophers,  physicists,  engineers, 
and  comic-strip  artists.  For  twenty-five  years  such  speculation 
was  unsupported  by  direct  experimental  evidence,  but  beginning 
about  1 930  such  evidence  began  to  appear  in  rapidly  increasing 
quantity.  Before  discussing  such  evidence  and  the  practical  partial 
conversion  of  matter  into  energy  that  is  our  main  theme,  we  shall 
review  the  foundations  of  atomic  and  nuclear  physics.  General 
familiarity  with  the  atomic  nature  of  matter  and  with  the  exist- 
ence of  electrons  is  assumed.  Our  treatment  will  be  little  more 
than  an  outline  which  may  be  elaborated  by  reference  to  books 
such  as  Pollard  and  Davidson's  Applied  Nuclear  Physics  and  Strana- 
than's  The  "Particles"  of  Modern  Physics. 


1.6.  First  discovered  by  H.  Becquerel  in  1896  and  subsequently 
studied  by  Pierre  and  Marie  Curie,  E.  Rutherford,  and  many 
others,  the  phe'nomena  of  radioactivity  have  played  leading  roles 
in  the  discovery  of  the  general  laws  of  atomic  structure  and  in  the 
verification  of  the  equivalence  of  mass  and  energy. 


1.7.  The  first  phenomenon  of  radioactivity  observed  was  the 
blackening  of  photographic  plates  by  uranium  minerals.   Al- 
though this  effect  is  still  used  to  some  extent  in  research  on  radio- 
activity, the  property  of  radioactive  substances  that  is  of  greatest 
scientific  value  is  their  ability  to  ionize  gases.  Under  normal  con- 
ditions air  and  other  gases  do  not  conduct  electricity — otherwise 
power  lines  and  electrical  machines  would  not  operate  in  the 

4  Introduction 

open  as  they  do.  But  under  some  circumstances  the  molecules 
of  air  are  broken  apart  into  positively  and  negatively  charged 
fragments,  called  ions.  Air  thus  ionized  does  conduct  electricity. 
Within  a  few  months  after  the  first  discovery  of  radioactivity 
Becquerel  found  that  uranium  had  the  power  to  ionize  air. 
Specifically  he  found  that  the  charge  on  an  electroscope  would 
leak  away  rapidly  through  the  air  if  some  uranium  salts  were 
placed  near  it.  (The  same  thing  would  happen  to  a  storage 
battery  if  sufficient  radioactive  material  were  placed  near  by.) 
Ever  since  that  time  the  rate  of  discharge  of  an  electroscope  has 
served  as  a  measure  of  intensity  of  radioactivity.  Furthermore, 
nearly  all  present-day  instruments  for  studying  radioactive  phe- 
nomena depend  on  this  ionization  effect  directly  or  indirectly. 
An  elementary  account  of  such  instruments,  notably  electro- 
scopes, Geiger-Muller  counters,  ionization  chambers,  and  Wilson 
cloud  chambers  is  given  in  Appendix  1. 


1.8.  Evidence  that  different  radioactive  substances  differ  in 
their  ionizing  power  both  in  kind  and  in  intensity  indicates  that 
there  are  differences  in  the  "radiations"  emitted.  Some  of  the 
radiations  are  much  more  penetrating  than  others;  consequently, 
two  radioactive  samples  having  the  same  effect  on  an   "un- 
shielded"  electroscope  may  have  very  different  effects  if  the 
electroscope  is  "shielded,"  i.e.,  if  screens  are  interposed  between 
the  sample  and  the  electroscope.  These  screens  are  said  to  absorb 
the  radiation. 

1.9.  Studies  of  absorption  and  other  phenomena  have  shown 
that  in  fact  there  are  three  types  of  "radiation"  given  off  by 
radioactive  substances.   There  are   alpha  particles,   which  are 
high-speed  ionized  helium  atoms  (actually  the  nuclei  of  helium 
atoms),    beta   particles,    which   are    high-speed   electrons,    and 
gamma  rays,   which  are  electromagnetic  radiations  similar  to 
X-rays.  Of  these  only  the  gamma  rays  are  properly  called  radia- 
tions, and  even  these  act  very  much  like  particles  because  of 
their  short  wave-length.  Such  a  "particle"  or  quantum  of  gamma 

Introduction  5 

radiation  is  called  a  photon.  In  general,  the  gamma  rays  are  very 
penetrating,  the  alpha  and  beta  rays  less  so.  Even  though  the 
alpha  and  beta  rays  are  not  very  penetrating,  they  have  enormous 
kinetic  energies  for  particles  of  atomic  size,  energies  thousands 
of  times  greater  than  the  kinetic  energies  which  the  molecules  of 
a  gas  have  by  reason  of  their  thermal  motion,  and  thousands 
of  times  greater  than  the  energy  changes  per  atom  in  chemical 
reactions.  It  was  for  this  reason  that  Einstein  suggested  that 
studies  of  radioactivity  might  show  the  equivalence  of  mass  and 


1.10.  Before  considering  what  types  of  atoms  emit  alpha,  beta 
and  gamma  rays,  and  before  discussing  the  laws  that  govern  such 
emission,  we  shall  describe  the  current  ideas  on  how  atoms  are 
constructed,  ideas  based  partly  on  the  study  of  radioactivity. 

1.11.  According  to  our  present  view  every  atom  consists  of  a 
small  heavy  nucleus  approximately  10~~12  cm  in  diameter  sur- 
rounded by  a  largely  empty  region  10~8  cm  in  diameter  in  which 
electrons  move  somewhat  like  planets  about  the  sun.  The  nucleus 
carries  an  integral  number  of  positive  charges,  each  1.6  X  10~19 
coulombs  in  size.   (See  Appendix  2  for  a  discussion  of  units.) 
Each  electron  carries  one  negative  charge  of  this  same  size,  and 
the  number  of  electrons  circulating  around  the  nucleus  is  equal 
to  the  number  of  positive  charges  on  the  nucleus  so  that  the  atom 
as  a  whole  has  a  net  charge  of  zero. 

1.12.  Atomic  Number  and  Electronic  Structure.  The  number  of 
positive  charges  in  the  nucleus  is  called  the  atomic  number,  Z. 
It  determines  the  number  of  electrons  in  the  extranuclear  struc- 
ture, and  this  in  turn  determines  the  chemical  properties  of  the 
atom.  Thus  all  the  atoms  of  a  given  chemical  element  have  the 
same  atomic  number,  and  conversely  all  atoms  having  the  same 
atomic  number  are  atoms  of  the  same  element  regardless  of 
possible  differences  in  their  nuclear  structure.  The  extranuclear 
electrons  in  an   atom   arrange   themselves  in  successive  shells 
according  to  well-established  laws.  Optical  spectra  arise  from 

6  Introduction 

disturbances  in  the  outer  parts  of  this  electron  structure;  X-rays 
arise  from  disturbances  of  the  electrons  close  to  the  nucleus.  The 
chemical  properties  of  an  atom  depend  on  the  outermost  elec- 
trons, and  the  formation  of  chemical  compounds  is  accompanied 
by  minor  rearrangements  of  these  electronic  structures.  Conse- 
quently, when  energy  is  obtained  by  oxidation,  combustion, 
explosion,  or  other  chemical  processes,  it  is  obtained  at  the 
expense  of  these  structures  so  that  the  arrangement  of  the  elec- 
trons in  the  products  of  the  process  must  be  one  of  lowered  energy 
content.  (Presumably  the  total  mass  of  these  products  is  corre- 
spondingly lower  but  not  detectably  so.)  The  atomic  nuclei  are 
not  affected  by  any  chemical  process. 

1.13.  Mass  Number.  Not  only  is  the  positive  charge  on  a  nucleus 
always  an  integral  number  of  electronic  charges,  but  the  mass 
of  the  nucleus  is  always  approximately  a  whole  number  times  a 
fundamental  unit  of  mass  which  is  almost  the  mass  of  a  proton, 
the  nucleus  of  a  hydrogen  atom.  (See  Appendix  2.)  This  whole 
number  is  called  the  mass  number,  A,  and  is  always  at  least  twice 
as  great  as  the  atomic  number  except  in  the  cases  of  hydrogen 
and  a  rare  isotope  of  helium.  Since  the  mass  of  a  proton  is  about 
1,800  times  that  of  an  electron,  the  mass  of  the  nucleus  is  very 
nearly  the  whole  mass  of  the  atom. 

1.14.  Isotopes  and  Isobars.  Two  species  of  atoms  having  the 
same  atomic  number  but  different  mass  numbers  are  called 
isotopes.  They  are  chemically  identical,  being  merely  two  species 
of  the  same  chemical  element.  If  two  species  of  atoms  have  the 
same  mass  number  but  different  atomic  numbers,  they  are  called 
isobars  and  represent  two  different  chemical  elements. 


1.15.  If  an  atom  emits  an  alpha  particle  (which  has  an  atomic 
number  of  two  and  a  mass  of  four),  it  becomes  an  atom  of  a 
different  element  with  an  atomic  number  lower  by  two  and  a 
mass  number  lower  by  four.  The  emission  by  a  nucleus  of  a  beta 
particle  increases  the  atomic  number  by  one  and  leaves  the  mass 
number  unaltered.  In  some  cases,  these  changes  are  accompanied 

Introduction  7 

by  the  emission  of  gamma  rays.  Elements  which  spontaneously 
change  or  "disintegrate"  in  these  ways  are  unstable  and  are 
described  as  being  "radioactive."  The  only  natural  elements 
which  exhibit  this  property  of  emitting  alpha  or  beta  particles 
are  (with  a  few  minor  exceptions)  those  of  very  high  atomic 
numbers  and  mass  numbers,  such  as  uranium,  thorium,  radium, 
and  actinium,  i.e.,  those  known  to  have  the  most  complicated 
nuclear  structures. 


1.16.  All  the  atoms  of  a  particular  radioactive  species  have  the 
same  probability  of  disintegrating  in  a  given  time,  so  that  an 
appreciable  sample  of  radioactive  material,   containing  many 
millions  of  atoms,  always  changes  or  "disintegrates"  at  the  same 
rate.  This  rate  at  which  the  material  changes  is  expressed  in 
terms  of  the  "half-life,"  the  time  required  for  one  half  the  atoms 
initially  present  to  disintegrate,  which  evidently  is  constant  for 
any  particular  atomic  species.  Half-lives  of  radioactive  materials 
range  from  fractions  of  a  second  for  the  most  unstable  to  billions 
of  years  for  those  which  are  only  slightly  unstable.  Often,  the 
"daughter"  nucleus  like  its  radioactive  "parent"  is  itself  radio- 
active and  so  on  down  the  line  for  several  successive  generations 
of  nuclei  until  a  stable  one  is  finally  reached.  There  are  three 
such  families  or  series  comprising  all  together  about  forty  different 
radioactive  species.  The  radium  series  starts  from  one  isotope  of 
uranium,  the  actinium  series  from  another  isotope  of  uranium, 
and  the  thorium  series  from  thorium.  The  final  product  of  each 
series,  after  ten  or  twelve  successive  emissions  of  alpha  and  beta 
particles,  is  a  stable  isotope  of  lead. 


1.17.  Before  1919  no  one  had  succeeded  in  disturbing  the 
stability  of  ordinary  nuclei  or  affecting  the  disintegration  rates 
of  those  that  were  naturally  radioactive.  In  1919  Rutherford 
showed  that  high-energy  alpha  particles  could  cause  an  alteration 

108 WAS  1VOIW3HO  ONV(Z)  H38WnN    OIW01V 

•3TJ-  fXlO  (M  O—  .-CO  «0> 

aw          zo>          oo>          aS>          HO>          <co 


Introduction  9 

in  the  nucleus  of  an  ordinary  element.  Specifically  he  succeeded 
in  changing  a  few  atoms  of  nitrogen  into  atoms  of  oxygen  by 
bombarding  them  with  alpha  particles.  The  process  involved 
may  be  written  as 

He4  +  N14  -i  O17  +  H1 

meaning  that  a  helium  nucleus  of  mass  number  4  (an  alpha 
particle)  striking  a  nitrogen  nucleus  of  mass  number  14  produces 
an  oxygen  nucleus  of  mass  number  17  and  a  hydrogen  nucleus 
of  mass  number  1.  The  hydrogen  nucleus,  known  as  the  "proton," 
is  of  special  importance  since  it  has  the  smallest  mass  of  any 
nucleus.  Although  protons  do  not  appear  in  natural  radioactive 
processes,  there  is  much  direct  evidence  that  they  can  be  knocked 
out  of  nuclei. 


1.18.  In  the  decade  following  Rutherford's  work  many  similar 
experiments  were  performed  with  similar  results.  One  series  of 
experiments  of  this  type  led  to  the  discovery  of  the  neutron, 
which  will  be  discussed  in  some  detail  since  the  neutron  is  prac- 
tically the  theme  song  of  this  whole  project. 

1.19.  In  1930  W.  Bothe  and  H.  Becker  in  Germany  found  that 
if  the  very  energetic  natural  alpha  particles  from  polonium  fell 
on  certain  of  the  light  elements,  specifically  beryllium,  boron  or 
lithium,  an  unusually  penetrating  radiation  was  produced.  At 
first  this  radiation  was  thought  to  be  gamma  radiation  although 
it  was  more  penetrating  than  any  gamma  rays  known,  and  the 
details  of  experimental  results  were  very  difficult  to  interpret  on 
this  basis.  The  next  important  contribution  was  reported  in  1932 
by  Irene  Curie  and  F.  Joliot  in  Paris.  They  showed  that  if  this 
unknown   radiation   fell   on   paraffin   or   any  other   hydrogen- 
containing  compound  it  ejected  protons  of  very  high  energy. 
This  was  not  in  itself  inconsistent  with  the  assumed  gamma-ray 
nature  of  the  new  radiation,  but  detailed  quantitative  analysis  of 
the  data  became  increasingly  difficult  to  reconcile  with  such  an 
hypothesis.  Finally  (later  in  1932)  J.  Chadwick  in  England  per- 

10  Introduction 

formed  a  series  of  experiments  showing  that  the  gamma-ray 
hypothesis  was  untenable.  He  suggested  that  in  fact  the  new 
radiation  consisted  of  uncharged  particles  of  approximately  the 
mass  of  the  proton,  and  he  performed  a  series  of  experiments 
verifying  his  suggestion.  Such  uncharged  particles  are  now  called 

1.20.  The  one  characteristic  of  neutrons  which  differentiates 
them  from  other  subatomic  particles  is  the  fact  that  they  are 
uncharged.  This  property  of  neutrons  delayed  their  discovery, 
makes  them  very  penetrating,  makes  it  impossible  to  observe 
them  directly,  and  makes  them  very  important  as  agents  in 
nuclear  change.  To  be  sure,  an  atom  in  its  normal  state  is  also 
uncharged,  but  it  is  ten  thousand  times  larger  than  a  neutron  and 
consists  of  a  complex  system  of  negatively  charged  electrons 
widely  spaced  around  a  positively  charged  nucleus.  Charged 
particles  (such  as  protons,  electrons,  or  alpha  particles)  and 
electromagnetic  radiations  (such  as  gamma  rays)  lose  energy  in 
passing  through  matter.  They  exert  electric  forces  which  ionize 
atoms  of  the  material  through  which  they  pass.  (It  is  such  ioniza- 
tion  processes  that  make  the  air  electrically  conducting  in  the 
path  of  electric  sparks  and  lightning  flashes.)  The  energy  taken 
up  in  ionization  equals  the  energy  lost  by  the  charged  particle, 
which  slows  down,  or  by  the  gamma  ray,  which  is  absorbed.  The 
neutron,  however,  is  unaffected  by  such  forces;  it  is  affected  only 
by  a  very  short-range  force,  i.e.,  a  force  that  comes  into  play 
when  the  neutron  comes  very  close  indeed  to  an  atomic  nucleus. 
This  is  the  kind  of  force  that  holds  a  nucleus  together  in  spite  of 
the  mutual  repulsion  of  the  positive  charges  in  it.  Consequently 
a  free  neutron  goes  on  its  way  unchecked  until  it  makes  a 
"head-on"  collision  with  an  atomic  nucleus.  Since  nuclei  are 
very  small,  such  collisions  occur  but  rarely  and  the  neutron 
travels  a  long  way  before  colliding.  In  the  case  of  a  collision  of 
the  "elastic"  type,  the  ordinary  laws  of  momentum  apply  as 
they  do  in  the  elastic  collision  of  billiard  balls.  If  the  nucleus  that 
is  struck  is  heavy,  it  acquires  relatively  little  speed,  but  if  it  is  a 
proton,  which  is  approximately  equal  in  mass  to  the  neutron, 

Introduction  1 1 

it  is  projected  forward  with  a  large  fraction  of  the  original  speed 
of  the  neutron,  which  is  itself  correspondingly  slowed.  Secondary 
projectiles  resulting  from  these  collisions  may  be  detected,  for 
they  are  charged  and  produce  ionization.  The  uncharged  nature 
of  the  neutron  makes  it  not  only  difficult  to  detect  but  difficult  to 
control.  Charged  particles  can  be  accelerated,  decelerated,  or 
deflected  by  electric  or  magnetic  fields  which  have  no  effect  on 
neutrons.  Furthermore,  free  neutrons  can  be  obtained  only  from 
nuclear  disintegrations;  there  is  no  natural  supply.  The  only 
means  we  have  of  controlling  free  neutrons  is  to  put  nuclei  in 
their  way  so  that  they  will  be  slowed  and  deflected  or  absorbed 
by  collisions.  As  we  shall  see,  these  effects  are  of  the  greatest 
practical  importance. 


1.21.  The  year  1932  brought  the  discovery  not  only  of  the 
neutron  but  also  of  the  positron.  The  positron  was  first  observed 
by  C.  D.  Anderson  at  the  California  Institute  of  Technology.  It 
has  the  same  mass  and  the  same  magnitude  of  charge  as  the 
electron,  but  the  charge  is  positive  instead  of  negative.  Except 
as  a  particle  emitted  by  artificially  radioactive  nuclei,  it  is  of 
little  interest  to  us. 

1.22.  One  other  major  discovery  marked  the  year  1932.  H.  C. 
Urey,  F.  G.  Brickwedde,  and  G.  M.  Murphy  found  that  hydrogen 
had  an  isotope  of  mass  number  2,  present  in  natural  hydrogen 
to  one  part  in  5,000.  Because  of  its  special  importance  this  heavy 
species  of  hydrogen  is  given  a  name  of  its  own,  deuterium,  and 
the  corresponding  nucleus  is  called  the  deuteron.  Like  the  alpha 
particle  the  deuteron  is  not  one  of  the  fundamental  particles  but 
does  play  an  important  role  in  certain  processes  for  producing 
nuclear  disintegration. 


1.23.  The  idea  that  all  elements  are  made  out  of  a  few  funda- 
mental particles  is  an  old  one.  It  is  now  firmly  established.  We 
believe  that  there  are  three  fundamental  particles — the  neutron, 

12  Introduction 

the  proton,  and  the  electron.  A  complete  treatise  would  also 
discuss  the  positron,  which  we  have  mentioned,  the  neutrino  and 
the  mesotron.  The  deuteron  and  alpha  particle,  which  have 
already  been  mentioned,  are  important  complex  particles. 

1.24.  According  to  our  present  views  the  nuclei  of  all  atomic 
species  are  made  up  of  neutrons  and  protons.  The  number  of 
protons  is  equal  to  the  atomic  number,  Z.  The  number  of  neu- 
trons, N,  is  equal  to  the  difference  between  the  mass  number  and 
the  atomic  number,  or  A  —  Z.  There  are  two  sets  of  forces 
acting  on  these  particles,  ordinary  electric  coulomb  forces  of 
repulsion  between  the  positive  charges  and  very  short-range 
forces  between  all  the  particles.  These  last  forces  are  only 
partly  understood,  and  we  shall  not  attempt  to  discuss  them. 
Suffice  it  to  say  that  combined  effects  of  these  attractive  and 
repulsive  forces  are  such  that  only  certain  combinations  of 
neutrons  and  protons  are  stable.  If  the  neutrons  and  protons  are 
few  in  number,  stability  occurs  when  their  numbers  are  about 
equal.  For  larger  nuclei,  the  proportion  of  neutrons  required  for 
stability  is  greater.  Finally,  at  the  end  of  the  periodic  table, 
where  the  number  of  protons  is  over  90  and  the  number  of 
neutrons  nearly  1 50,  there  are  no  completely  stable  nuclei.  (Some 
of  the  heavy  nuclei  are  almost  stable  as  evidenced  by  very  long 
half-lives.)  If  an  unstable  nucleus  is  formed  artificially  by  adding 
an  extra  neutron  or  proton,  eventually  a  change  to  a  stable  form 
occurs.  Strangely  enough,  this  is  not  accomplished  by  ejecting  a 
proton  or  a  neutron  but  by  ejecting  a  positron  or  an  electron; 
apparently  within  the  nucleus  a  proton  converts  itself  into  a 
neutron  and  positron  (or  a  neutron  converts  itself  into  a  proton 
and  electron),  and  the  light  charged  particle  is  ejected.  In  other 
words,  the  mass  number  remains  the  same  but  the  atomic  number 
changes.  The  stability  conditions  are  not  very  critical  so  that  for 
a  given  mass  number,  i.e.,  given  total  number  of  protons  and 
neutrons,  there  may  be  several  stable  arrangements  of  protons 
and  neutrons  (at  most  three  or  five)  giving  several  isobars.  For  a 
given  atomic  number,  i.e.,  given  number  of  protons,  conditions 
can  vary  still  more  widely  so  that  some  of  the  heavy  elements 

Introduction  13 

have  as  many  as  ten  or  twelve  stable  isotopes.  Some  two  hundred 
and  fifty  different  stable  nuclei  have  been  identified,  ranging  in 
mass  number  from  one  to  two  hundred  and  thirty-eight  and  in 
atomic  number  from  one  to  ninety-two. 

1.25.  All  the  statements  we  have  been  making  are  based  on 
experimental   evidence.    The   theory   of  nuclear   forces   is   still 
incomplete,  but  it  has  been  developed  on  quantum-mechanical 
principles  sufficiently  to  explain  not  only  the  above  observations 
but  more  detailed  empirical  data  on  artificial  radioactivity  and 
on  differences  between  nuclei  with  odd  and  even  mass  numbers. 


1.26.  We   mentioned  the  emission  of  positrons  or  electrons 
by  nuclei  seeking  stability.   Electron  emission   (beta  rays)  was 
already    familiar    in    the   study   of  naturally   radioactive   sub- 
stances, but  positron  emission  was  not  found  in  the  case  of  such 
substances.    In   fact,    the    general    discussion    presented    above 
obviously  was  based  in  part  on  information  that  cannot  be  pre- 
sented in  this  report.  We  shall,  however,  give  a  brief  account  of 
the  discovery  of  "artificial"  radioactivity  and  what  is  now  known 
about  it. 

1.27.  In  1934,  Curie  and  Joliot  reported  that  certain  light 
elements  (boron,  magnesium,  aluminum)  which  had  been  bom- 
barded with  alpha  particles  continued  to  emit  positrons  for  some 
time   after   the   bombardment   was   stopped.    In   other   words, 
alpha-particle    bombardment    produced    radioactive    forms    of 
boron,  magnesium,  and  aluminum.  Curie  and  Joliot  actually 
measured  half-lives  of  14  minutes,  2.5  minutes,  and  3.25  minutes, 
respectively,  for  the  radioactive  substances  formed  by  the  alpha- 
particle  bombardment. 

1.28.  This  result  stimulated  similar  experiments  all  over  the 
world.  In  particular,  E.  Fermi  reasoned  that  neutrons,  because 
of  their  lack  of  charge,  should  be  effective  in  penetrating  nuclei, 
especially  those  of  high  atomic  number  which  repel  protons  and 
alpha  particles  strongly.  He  was  able  to  verify  his  prediction 
almost  immediately,  finding  that  the  nucleus  of  the  bombarded 

14  Introduction 

atom  captured  the  neutron  and  that  there  was  thus  produced  an 
unstable  nucleus  which  then  achieved  stability  by  emitting  an 
electron.  Thus,  the  final,  stable  nucleus  was  one  unit  higher  in 
mass  number  and  one  unit  higher  in  atomic  number  than  the 
initial  target  nucleus. 

1.29.  As  a  result  of  innumerable  experiments  carried  out  since 
1934,  radioactive  isotopes  of  nearly  every  element  in  the  periodic 
table  can  now  be  produced.  Some  of  them  revert  to  stability  by 
the  emission  of  positrons,  some  by  the  emission  of  electrons, 
some  by  a  process  known  as  K-electron  capture  which  we  shall 
not  discuss,  and  a  small  number  (probably  three)  by  alpha- 
particle  emission.  Altogether  some  five  hundred  unstable  nuclear 
species  have  been  observed,   and  in  most  cases  their  atomic 
numbers  and  mass  numbers  have  been  identified. 

1.30.  Not  only  do  these  artificially  radioactive  elements  play 
an  important  role  throughout  the  project  with  which  we  are 
concerned,    but   their   future   value    in   medicine,    in   "tracer" 
chemistry,  and  in  many  other  fields  of  research  can  hardly  be 



1.31.  In  describing  radioactivity  and  atomic  structure  we  have 
deliberately  avoided  quantitative  data  and  have  not  mentioned 
any  applications  of  the  equivalence  of  mass  and  energy  which  we 
announced  as  the  guiding  principle  of  this  report.  Now  when 
we  must  speak  of  quantitative  details,  not  merely  of  general 

1.32.  We  have  spoken  of  stable  and  unstable  nuclei  made  up 
of  assemblages  of  protons  and  neutrons  held  together  by  nuclear 
forces.  It  is  a  general  principle  of  physics  that  work  must  be 
done  on  a  stable  system  to  break  it  up.  Thus,  if  an  assemblage  of 
neutrons  and  protons  is  stable,  energy  must  be  supplied  to  sepa- 
rate  its   constituent  particles.    If  energy  and   mass   are   really 
equivalent,  then  the  total  mass  of  a  stable  nucleus  should  be  less 

Introduction  1 5 

than  the  total  mass  of  the  separate  protons  and  neutrons  that 
go  to  make  it  up.  This  mass  difference,  then,  should  be  equivalent 
to  the  energy  required  to  disrupt  the  nucleus  completely,  which 
is  called  the  binding  energy.  Remember  that  the  masses  of  all 
nuclei  were  "approximately"  whole  numbers.  It  is  the  small 
differences  from  whole  numbers  that  are  significant. 

1.33.  Consider  the  alpha  particle  as  an  example.  It  is  stable; 
since  its  mass  number  is  four  and  its  atomic  number  two  it  con- 
sists of  two  protons  and  two  neutrons.  The  mass  of  a  proton  is 
1.00758  and  that  of  a  neutron  is  1.00893  (see  Appendix  2),  so 
that  the  total  mass  of  the  separate  components  of  the  helium 
nucleus  is 

2  X  1.00758  -f  2  X  1.00893  =  4.03302 

whereas  the  mass  of  the  helium  nucleus  itself  is  4.00280.  Neglect- 
ing the  last  two  decimal  places  we  have  4.033  and  4.003,  a 
difference  of  0.030  mass  units.  This,  then,  represents  the  "binding 
energy"  of  the  protons  and  neutrons  in  the  helium  nucleus.  It 
looks  small,  but  recalling  Einstein's  equation,  E  =  me2,  we  re- 
member that  a  small  amount  of  mass  is  equivalent  to  a  large 
amount  of  energy.  Actually  0.030  mass  units  is  equal  to  4. 5  X  10~5 
ergs  per  nucleus  or  2.7  X  1019  ergs  per  gram  molecule  of  helium. 
In  units  more  familiar  to  the  engineer  or  chemist,  this  means  that 
to  break  up  the  nuclei  of  all  the  helium  atoms  in  a  gram  of  helium 
would  require  1.62  X  1011  gram  calories  or  190,000  kilowatt 
hours  of  energy.  Conversely,  if  free  protons  and  neutrons  could 
be  assembled  into  helium  nuclei,  this  energy  would  be  released. 

1.34.  Evidently  it  is  worth  exploring  the  possibility  of  getting 
energy  by  combining  protons  and  neutrons  or  by  transmuting 
one  kind  of  nucleus  into  another.  Let  us  begin  by  reviewing 
present-day  knowledge  of  the  binding  energies  of  various  nuclei. 


1.35.  Chemical  atomic-weight  determinations  give  the  average 
weight  of  a  large  number  of  atoms  of  a  given  element.  Unless  the 
element  has  only  one  isotope,  the  chemical  atomic  weight  is  not 

1 6  Introduction 

proportional  to  the  mass  of  individual  atoms.  The  mass  spectro- 
graph  developed  by  F.  W.  Aston  and  others  from  the  earlier 
apparatus  of  J.  J.  Thomson  measures  the  masses  of  individual 
isotopes.  Indeed,  it  was  just  such  measurements  that  proved  the 
existence  of  isotopes  and  showed  that  on  the  atomic-weight  scale 
the  masses  of  all  atomic  species  were  very  nearly  whole  numbers. 
These  whole  numbers,  discovered  experimentally,  are  the  mass 
numbers  which  we  have  already  defined  and  which  represent 
the  sums  of  the  numbers  of  the  protons  and  neutrons;  their 
discovery  contributed  largely  to  our  present  views  that  all  nuclei 
are  combinations  of  neutrons  and  protons. 

1.36.  Improved  mass  spectrograph  data  supplemented  in  a 
few  cases  by  nuclear  reaction  data  have  given  accurate  figures 
for  binding  energies  for  many  atomic  species  over  the  whole 
range  of  atomic  masses.  This  binding  energy,  B,  is  the  difference 
between  the  true  nuclear  mass,  M,  and  the  sum  of  the  masses 
of  all  the  protons  and  neutrons  in  the  nucleus.  That  is, 

B  =  (ZMP  +  NMn)  -  M 

where  Mp  and  Mn  are  the  masses  of  the  proton  and  neutron 
respectively,  Z  is  the  number  of  protons,  N  =  A  —  Z  is  the  num- 
ber of  neutrons,  and  M  is  the  true  mass  of  the  nucleus.  It  is  more 
interesting  to  study  the  binding  energy  per  particle,  B/A,  than 
B  itself.  Such  a  study  shows  that,  apart  from  fluctuations  in  the 
light  nuclei,  the  general  trend  of  the  binding  energy  per  particle 
is  to  increase  rapidly  to  a  flat  maximum  around  A  =  60  (nickel) 
and  then  decrease  again  gradually.  Evidently  the  nuclei  in  the 
middle  of  the  periodic  table — nuclei  of  mass  numbers  40  to  100 — 
are  the  most  strongly  bound.  Any  nuclear  reaction  where  the 
particles  in  the  resultant  nuclei  are  more  strongly  bound  than 
the  particles  in  the  initial  nuclei  will  release  energy.  Speaking  in 
thermochemical  terms,  such  reactions  are  exothermic.  Thus,  in 
general,  energy  may  be  gained  by  combining  light  nuclei  to 
form  heavier  ones  or  by  breaking  very  heavy  ones  into  two  or 
three  smaller  fragments.  Also,  there  are  a  number  of  special 
cases  of  exothermic  nuclear  disintegrations  among  the  first  ten 

Introduction  1 7 

or  twelve  elements  of  the  periodic  table,  where  the  binding 
energy  per  particle  varies  irregularly  from  one  element  to  another. 

1.37.  So  far  we  seem  to  be  piling  one  supposition  on  another. 
First  we  assumed  that  mass  and  energy  were  equivalent;  now  we 
are   assuming   that   atomic   nuclei  can   be  rearranged   with  a 
consequent  reduction  in  their  total  mass,  thereby  releasing  energy 
which  can  then  be  put  to  use.  It  is  time  to  talk  about  some  ex- 
periments that  convinced  physicists  of  the  truth  of  these  statements. 


1.38.  As  we  have  already  said,  Rutherford's  work  in  1919  on 
artificial  nuclear  disintegration  was  followed  by  many  similar 
experiments.   Gradual  improvement  in   high-voltage  technique 
made  it  possible  to  substitute  artificially  produced  high-speed 
ions  of  hydrogen  or  helium  for  natural  alpha  particles.  J.  D. 
Cockcroft  and  E.  T.  S.  Walton  in  Rutherford's  laboratory  were 
the  first  to  succeed  in  producing  nuclear  changes  by  such  methods. 
In  1932  they  bombarded  a  target  of  lithium  with  protons  of  700 
kilovolts  energy  and  found  that  alpha  particles  were  ejected  from 
the  target  as  a  result  of  the  bombardment.  The  nuclear  reaction 
which  occurred  can  be  written  symbolically  as 

3Li7  +  iH1  -»  2He4  +  2He4 

where  the  subscript  represents  the  positive  charge  on  the  nucleus 
(atomic  number)  and  the  superscript  is  the  number  of  massive 
particles  in  the  nucleus  (mass  number).  As  in  a  chemical  equa- 
tion, quantities  on  the  left  must  add  up  to  those  on  the  right;  thus 
the  subscripts  total  four  and  the  superscripts  eight  on  each  side. 

1.39.  Neither  mass  nor  energy  has  been  included  in  this  equa- 
tion.  In  general,  the  incident  proton  and  the  resultant  alpha 
particles  will  each  have  kinetic  energy.  Also,  the  mass  of  two 
alpha  particles  will  not  be  precisely  the  same  as  the  sum  of  the 
masses  of  a  proton  and  a  lithium  atom.  According  to  our  theory, 
the  totals  of  mass  and  energy  taken  together  should  be  the  same 
before  and  after  the  reaction.  The  masses  were  known  from  mass 

1 8  Introduction 

spectra.  On  the  left  (Li7  -f  H1)  they  totalled  8.0241,  on  the  right 
(2He4)  8.0056,  so  that  0.0185  units  of  mass  had  disappeared  in 
the  reaction.  The  experimentally  determined  energies  of  the 
alpha  particles  were  approximately  8.5  million  electron  volts 
each,  a  figure  compared  to  which  the  kinetic  energy  of  the  inci- 
dent proton  could  be  neglected.  Thus  0.0185  units  of  mass  had 
disappeared  and  17  Mev  of  kinetic  energy  had  appeared.  Now 
0.0185  units  of  mass  is  3.07  X  10~26  grams,  17  Mev  is  27.2  X  10~6 
ergs  and  c  is  3  X  1010  cm/sec.  (See  Appendix  2.)  If  we  substitute 
these  figures  into  Einstein's  equation,  E  =  me2,  on  the  left  side 
we  have  27.2  X  10~6  ergs  and  on  the  right  side  we  have  27.6  X 
10~6  ergs,  so  that  the  equation  is  found  to  be  satisfied  to  a  good 
approximation.  In  other  words,  these  experimental  results  prove 
that  the  equivalence  of  mass  and  energy  was  correctly  stated  by 



1.40.  Cockcroft  and  Walton  produced  protons  of  fairly  high 
energy  by  ionizing  gaseous  hydrogen  and  then  accelerating  the 
ions  in  a  transformer-rectifier  high-voltage  apparatus.  A  similar 
procedure  can  be  used  to  produce  high-energy  deuterons  from 
deuterium  or  high-energy  alpha  particles  from  helium.  Higher 
energies  can  be  attained  by  accelerating  the  ions  in  cyclotrons 
or  Van  de  Graaff  machines.  However,  to  obtain  high-energy 
gamma  radiation  or — most  important  of  all — to  obtain  neu- 
trons, nuclear  reactions  themselves  must  be  used  as  sources. 
Radiations  of  sufficiently  high  energy  come  from  certain  naturally 
radioactive  materials  or  from  certain  bombardments.  Neutrons 
are  commonly  produced  by  the  bombardment  of  certain  elements, 
notably  beryllium  or  boron,  by  natural  alpha  particles,  or  by 
bombarding  suitable  targets  with  protons  or  deuterons.  The  most 
common  source  of  neutrons  is  a  mixture  of  radium  and  beryllium 
where  the  alpha  particles  from  radium  and  its  decay  products 
penetrate  the  Be9  nuclei,  which  then  give  off  neutrons  and  become 

Introduction  1 9 

stable  C12  nuclei  (ordinary  carbon).  A  frequently  used  "beam" 
source  of  neutrons  results  from  accelerated  deuterons  impinging 
on  "heavy  water"  ice.  Here  the  high-speed  deuterons  strike  the 
target  deuterons  to  produce  neutrons  and  He3  nuclei.  Half  a 
dozen  other  reactions  are  also  used  involving  deuterium,  lithium, 
beryllium,  or  boron  as  targets.  Note  that  in  all  these  reactions  the 
total  mass  number  and  total  charge  number  are  unchanged. 

1.41.  To  summarize,   the  agents  that  are  found  to  initiate 
nuclear  reactions  are — in  approximate  order  of  importance — 
neutrons,  deuterons,  protons,  alpha  particles,  gamma  rays  and, 
rarely,  heavier  particles. 


1.42.  Most  atomic  nuclei  can  be  penetrated  by  at  least  one 
type  of  atomic  projectile  (or  by  gamma  radiation).  Any  such 
penetration  may  result  in  a  nuclear  rearrangement  in  the  course 
of  which  a  fundamental  particle  is  ejected  or  radiation  is  emitted 
or  both.  The  resulting  nucleus  may  be  one  of  the  naturally  avail- 
able stable  species,  or — more  likely — it  may  be  an  atom  of  a 
different  type  which  is  radioactive,  eventually  changing  to  still  a 
different  nucleus.  This  may  in  turn  be  radioactive  and,  if  so,  will 
again  decay.  The  process  continues  until  all  nuclei  have  changed 
to  a  stable  type.  There  are  two  respects  in  which  these  artificially 
radioactive  substances  differ  from  the  natural  ones:   many  of 
them  change  by  emitting  positrons  (unknown  in  natural  radio- 
activity) and  very  few  of  them  emit  alpha  particles.  In  every  one 
of  the  cases  where  accurate  measurements  have  been  made,  the 
equivalence  of  mass  and  energy  has  been  demonstrated  and  the 
mass-energy  total  has  remained  constant.  (Sometimes  it  is  neces- 
sary to  invoke  neutrinos  to  preserve  mass-energy  conservation.) 


1.43.  A  complete  description  of  a  nuclear  reaction  should 
include  the  nature,  mass  and  energy  of  the  incident  particle,  also 
the  nature  (mass  number  and  atomic  number),  mass  and  energy 
(usually  zero)  of  the  target  particle,  also  the  nature,  mass  and 

20  Introduction 

energy  of  the  ejected  particles  (or  radiation),  and  finally  the 
nature,  mass  and  energy  of  the  remainder.  But  all  of  these  are 
rarely  known  and  for  many  purposes  their  complete  specification 
is  unnecessary.  A  nuclear  reaction  is  frequently  described  by  a 
notation  that  designates  first  the  target  by  chemical  symbol  and 
mass  number  if  known,  then  the  projectile,  then  the  emitted 
particle,  and  then  the  remainder.  In  this  scheme  the  neutron 
is  represented  by  the  letter  n,  the  proton  by  p,  the  deuteron  by  d, 
the  alpha  particle  by  <x,  and  the  gamma  ray  by  7.  Thus  the 
radium-beryllium  neutron  reaction  can  be  written  Be2(a,  n)C12 
and  the  deuteron-deuteron  reaction  H2(d,  n)He3. 


1.44.  Considering  the  five  different  particles  (n,  p.  d,  a,  7) 
both  as  projectiles  and  emitted  products,  we  might  expect  to 
find  twenty-five  combinations  possible.  Actually  the  deuteron 
very  rarely  occurs  as  a  product  particle,  and  the  photon  initiates 
only  two  types  of  reaction.  There  are,  however,  a  few  other  types 
of  reaction,  such  as  (n,  2n),  (d,  H3),  and  fission,  which  bring  the 
total  known  types  to  about  twenty-five.  Perhaps  the  (n,  7)  reac- 
tion should  be  specifically  mentioned  as  it  is  very  important  in 
one  process  which  will  concern  us.  It  is  often  called  "radiative 
capture"  since  the  neutron  remains  in  the  nucleus  and  only  a 
gamma  ray  comes  out. 


1.45.  So  far  nothing  has  been  said  about  the  probability  of 
nuclear  reactions.  Actually  it  varies  widely.  There  is  no  guarantee 
that  a  neutron  or  proton  headed  straight  for  a  nucleus  will 
penetrate  it  at  all.  It  depends  on  the  nucleus  and  on  the  incident 
particle.  In  nuclear  physics,  it  is  found  convenient  to  express 
probability  of  a  particular  event  by  a  "cross  section."  Statistically, 
the  centers  of  the  atoms  in  a  thin  foil  can  be  considered  as  points 
evenly  distributed  over  a  plane.  The  center  of  an  atomic  projec- 
tile striking  this  plane  has  geometrically  a  definite  probability  of 

Introduction  21 

passing  within  a  certain  distance  (r)  of  one  of  these  points.  In  fact, 
if  there  are  n  atomic  centers  in  an  area  A  of  the  plane,  this  prob- 
ability is  nrr2/A,  which  is  simply  the  ratio  of  the  aggregate  area 
of  circles  of  radius  r  drawn  around  the  points  to  the  whole  area. 
If  we  think  of  the  atoms  as  impenetrable  steel  discs  and  the 
impinging  particle  as  a  bullet  of  negligible  diameter,  this  ratio 
is  the  probability  that  the  bullet  will  strike  a  steel  disc,  i.e.,  that 
the  atomic  projectile  will  be  stopped  by  the  foil.  If  it  is  the  fraction 
of  impinging  atoms  getting  through  the  foil  which  is  measured,  the 
result  can  still  be  expressed  in  terms  of  the  equivalent  stopping 
cross  section  of  the  atoms.  This  notion  can  be  extended  to  any 
interaction  between  the  impinging  particle  and  the  atoms  in  the 
target.  For  example,  the  probability  that  an  alpha  particle  strik- 
ing a  beryllium  target  will  produce  a  neutron  can  be  expressed 
as  the  equivalent  cross  section  of  beryllium  for  this  type  of 

1.46.  In  nuclear  physics  it  is  conventional  to  consider  that  the  impinging 
particles  have  negligible  diameter.  The  technical  definition  of  cross  section 
for  any  nuclear  process  is  therefore: 

number  of 

processes  occurring 
,    — 7- =  (number  of  target  nuclei  per  cm2)  X  (nuclear 

incident  particles  CTOSS  scction  in  cm2) 

It  should  be  noted  that  this  definition  is  for  the  cross  section  per  nucleus. 
Gross  sections  can  be  computed  for  any  sort  of  process,  such  as  capture 
scattering,  production  of  neutrons,  etc.  In  many  cases,  the  number  of  parti- 
cles emitted  or  scattered  in  nuclear  processes  is  not  measured  directly;  one 
merely  measures  the  attenuation  produced  in  a  parallel  beam  of  incident 
particles  by  the  interposition  of  a  known  thickness  of  a  particular  material. 
The  cross  section  obtained  in  this  way  is  called  the  total  cross  section  and  is 
usually  denoted  by  <r. 

1.47.  As  indicated  in  paragraph  1.11,  the  typical  nuclear  diameter  is  of 
the  order  of  10~12  cm.  We  might  therefore  expect  the  cross  sections  for  nuclear 
reactions  to  be  of  the  order  of  xd2/4  or  roughly  10~24  cm2  and  this  is  the  unit 
in  which  they  are  usually  expressed.  Actually  the  observed  cross  sections 
vary  enormously.  Thus  for  slow  neutrons  absorbed  by  the  (n,  7)  reaction  the 
cross  section  in  some  cases  is  as  much  as  1,000  X  10~24  cm2,  while  the  cross 
sections  for  transmutations  by  gamma-ray  absorption  are  in  the  neighbor- 
hood of  (1/1,000)  X  10~24  cm2. 

22  Introduction 

IN  1939 


1.48.  We  have  talked  glibly  about  the  equivalence  of  mass 
and  energy  and  about  nuclear  reactions,  such  as  that  of  protons 
on  lithium,  where  energy  was  released  in  relatively  large  amounts. 
Now  let  us  ask  why  atomic  power  plants  did  not  spring  up  all 
over  the  world  in  the  'thirties.  After  all,  if  we  can  get  2.76  X  10~5 
ergs  from  an  atom  of  lithium  struck  by  a  proton,  we  might  expect 
to  obtain  approximately  half  a  million  kilowatt  hours  by  com- 
bining a  gram  of  hydrogen  with  seven  grams  of  lithium.  It  looks 
better  than  burning  coal.  The  difficulties  are  in  producing  the 
high-speed  protons  and  in  controlling  the  energy  produced.  All 
the  experiments  we  have  been  talking  about  were  done  with 
very  small  quantities  of  material,  large  enough  in  numbers  of 
atoms,  to  be  sure,  but  in  terms  of  ordinary  masses  infinitesimal — 
not  tons  or  pounds  or  grams,  but  fractions  of  micrograms.  The 
amount  of  energy  used  up  in  the  experiment  was  always  far 
greater  than  the  amount  generated  by  the  nuclear  reaction. 

1.49.  Neutrons  are  particularly  effective  in  producing  nuclear 
disintegration.  Why  weren't  they  used?  If  their  initial  source  was 
an  ion  beam  striking  a  target,  the  limitations  discussed  in  the  last 
paragraph  applied.  If  a  radium  and  beryllium  source  was  to  be 
used,  the  scarcity  of  radium  was  a  difficulty. 


1.50.  Our  common  sources  of  power,  other  than  sunlight  and 
water  power,  are  chemical  reactions — usually  the  combustion  of 
coal  or  oil.  They  release  energy  as  the  result  of  rearrangements 
of  the  outer  electronic  structures  of  the  atoms,  the  same  kind  of 
process  that  supplies  energy  to  our  bodies.  Combustion  is  always 
self-propagating;  thus  lighting  a  fire  with  a  match  releases  enough 
heat  to  ignite  the  neighboring  fuel,  which  releases  more  heat  which 
ignites  more  fuel,  and  so  on.  In  the  nuclear  reactions  we  have 

Introduction  23 

described  this  is  not  generally  true;  neither  the  energy  released 
nor  the  new  particles  formed  are  sufficient  to  maintain  the  reac- 
tion. But  we  can  imagine  nuclear  reactions  emitting  particles  of 
the  same  sort  that  initiate  them  and  in  sufficient  numbers  to 
propagate  the  reaction  in  neighboring  nuclei.  Such  a  self- 
propagating  reaction  is  called  a  "chain  reaction"  and  such  con- 
ditions must  be  achieved  if  the  energy  of  the  nuclear  reactions 
with  which  we  are  concerned  is  to  be  put  to  large-scale  use. 


1.51.  Although  there  were  no  atomic  power  plants  built  in 
the  'thirties,  there  were  plenty  of  discoveries  in  nuclear  physics 
and  plenty  of  speculation.  A  theory  was  advanced  by  H.  Bethe 
to  explain  the  heat  of  the  sun  by  a  cycle  of  nuclear  changes 
involving  carbon,  hydrogen,  nitrogen,  and  oxygen,  and  leading 
eventually  to  the  formation  of  helium.*  This  theory  is  now 
generally  accepted.  The  discovery  of  a  few  (n,  2n)  nuclear  reac- 
tions (i.e.,  neutron-produced  and  neutron-producing  reactions) 
suggested  that  a  self-multiplying  chain  reaction  might  be  initiated 
under  the  right  conditions.  There  was  much  talk  of  atomic  power 
and  some  talk  of  atomic  bombs.  But  the  last  great  step  in  this 
preliminary  period  came  after  four  years  of  stumbling.  The 
effects  of  neutron  bombardment  of  uranium,  the  most  complex 
element  known,  had  been  studied  by  some  of  the  ablest  physicists. 
The  results  were  striking  but  confusing.  The  story  of  their  gradual 
interpretation  is  intricate  and  highly  technical,  a  fascinating  tale 
of  theory  and  experiment.  Passing  by  the  earlier  inadequate 

*  The  series  of  reactions  postulated  was 

(1)  eC1J  +  iH*-+7Ni3 

(2)  7N"  -*  6C13  +  1C« 

(3)  6C13+'iHi->7N" 

(4)  TN^  +  iH1-*^15 

(5)  8015  ->  TN»  +  ie« 

(6)  7N15  +  jHl  -» eG"  +  2He« 

The  net  effect  is  the  transformation  of  hydrogen  into  helium  and  positrons 
(designated  as  ie°)  and  the  release  of  about  thirty  million  electron  volts 

24  Introduction 

explanations,  we  shall  go  directly  to  the  final  explanation,  which, 
as  so  often  happens,  is  relatively  simple. 


1.52.  As  has  already  been  mentioned,  the  neutron  proved  to 
be   the  most  effective   particle  for  inducing  nuclear  changes. 
This  was  particularly  true  for  the  elements  of  highest  atomic 
number   and   weight   where   the   large   nuclear   charge   exerts 
strong  repulsive  forces  on  deuteron  or  proton  projectiles  but  not 
on  uncharged  neutrons.   The  results  of  the  bombardment  of 
uranium  by  neutrons  had  proved  interesting  and  puzzling.  First 
studied  by  Fermi  and  his  colleagues  in  1934,  they  were  not 
properly  interpreted  until  several  years  later. 

1.53.  On  January  16,  1939,  Niels  Bohr  of  Copenhagen,  Den- 
mark, arrived  in  this  country  to  spend  several  months  in  Prince- 
ton, N.  J.,  and  was  particularly  anxious  to  discuss  some  abstract 
problems  with  Einstein.   (Four  years  later  Bohr  was  to  escape 
from  Nazi-occupied  Denmark  in  a  small  boat.)  Just  before  Bohr 
left  Denmark  two  of  his  colleagues,  O.  R.  Frisch  and  L.  Meitner 
(both  refugees  from  Germany),  had  told  him  their  guess  that  the 
absorption  of  a  neutron  by  a  uranium  nucleus  sometimes  caused 
that  nucleus  to  split  into  approximately  equal  parts  with  the 
release  of  enormous  quantities  of  energy,  a  process  that  soon 
began   to   be  called   nuclear   "fission."   The   occasion   for   this 
hypothesis  was  the  important  discovery  of  O.   Hahn  and  F. 
Strassmann  in  Germany  (published  in  Naturwissenschaften  in  early 
January  1939)  which  proved  that  an  isotope  of  barium  was 
produced  by  neutron  bombardment  of  uranium.  Immediately 
on  arrival  in  the  United  States  Bohr  communicated  this  idea  to 
his  former  student  J.  A.  Wheeler  and  others  at  Princeton,  and 
from  them  the  news  spread  by  word  of  mouth  to  neighboring 
physicists  including  E.  Fermi  at  Columbia  University.  As  a  result 
of  conversations    among    Fermi,  J.    R.    Dunning,    and   G.   B. 
Pegram,  a  search  was  undertaken  at  Columbia  for  the  heavy 
pulses  of  ionization  that  would  be  expected  from   the  flying 
fragments  of  the  uranium  nucleus.  On  January  26,  1939,  there 

Introduction  25 

was  a  conference  on  theoretical  physics  at  Washington,  D.  C., 
sponsored  jointly  by  the  George  Washington  University  and  the 
Carnegie  Institution  of  Washington.  Fermi  left  New  York  to 
attend  this  meeting  before  the  Columbia  fission  experiments  had 
been  tried.  At  the  meeting  Bohr  and  Fermi  discussed  the  problem 
of  fission,  and  in  particular  Fermi  mentioned  the  possibility  that 
neutrons  might  be  emitted  during  the  process.  Although  this 
was  only  a  guess,  its  implication  of  the  possibility  of  a  chain 
reaction  was  obvious.  A  number  of  sensational  articles  were  pub- 





lished  in  the  press  on  this  subject.  Before  the  meeting  in  Washing- 
ton was  over,  several  other  experiments  to  confirm  fission  had  been 
initiated,  and  positive  experimental  confirmation  was  reported 
from  four  laboratories  (Columbia  University,  Carnegie  Institu- 
tion of  Washington,  Johns  Hopkins  University,  University  of 
California)  in  the  February  15,  1939,  issue  of  the  Physical  Review. 
By  this  time  Bohr  had  heard  that  similar  experiments  had  been 
made  in  his  laboratory  in  Copenhagen  about  January  15.  (Letter 
by  Frisch  to  Nature  dated  January  16,  1939,  and  appearing  in  the 
February  18  issue.)  F.  Joliot  in  Paris  had  also  published  his  first 
results  in  the  Comptes  Rendus  of  January  30,  1939.  From  this  time 
on  there  was  a  steady  flow  of  papers  on  the  subject  of  fission,  so 
that  by  the  time  (December  6.  1939)  L.  A.  Turner  of  Princeton 

26  Introduction 

wrote  a  review  article  on  the  subject  in  the  Reviews  of  Modern 
Physics  nearly  one  hundred  papers  had  appeared.  Complete 
analysis  and  discussion  of  these  papers  have  appeared  in  Turner's 
article  and  elsewhere. 


1 .54.  Consider  the  suggestion  of  Frisch  and  Meitner  in  the  light 
of  the  two  general  trends  that  had  been  discovered  in  nuclear 
structure:  first,  that  the  proportion  of  neutrons  goes  up  with 
atomic  number;  second,  that  the  binding  energy  per  particle 
is  a  maximum  for  the  nuclei  of  intermediate  atomic  number. 
Suppose  the   U-238   nucleus  is  broken  exactly  in  half;   then, 
neglecting  the  mass  of  the  incident  neutron,  we  have  two  nuclei 
of  atomic  number  46  and  mass  number  119.  But  the  heaviest 
stable  isotope  of  palladium  (Z  =  46)  has  a  mass  number  of  only 
110.  Therefore  to  reach  stability  each  of  these  imaginary  new 
nuclei  must  eject  nine  neutrons,   becoming  4ePd110  nuclei;  or 
four  neutrons  in  each  nucleus  must  convert  themselves  to  protons 
by  emitting  electrons,  thereby  forming  stable  tin  nuclei  of  mass 
number  119  and  atomic  number  50;  or  a  combination  of  such 
ejections  and  conversions  must  occur  to  give  some  other  pair  of 
stable  nuclei.  Actually,  as  was  suggested  by  Hahn  and  Strass- 
mann's  identification  of  barium  (Z  =  56,  A  =  135  to  140)  as  a 
product  of  fission,  the  split  occurs  in  such  a  way  as  to  produce 
two  unequal  parts  of  mass  numbers  about  140  and  90  with  the 
emission  of  a  few  neutrons  and  subsequent  radioactive  decay  by 
electron  emission  until  stable  nuclei  are  formed.   Calculations 
from  binding-energy  data  show  that  any  such  rearrangement 
gives  an  aggregate  resulting  mass  considerably  less  than  the 
initial  mass  of  the  uranium  nucleus,  and  thus  that  a  great  deal 
of  energy  must  be  released. 

1.55.  Evidently,  there  were  three  major  implications  of  the 
phenomenon  of  fission:  the  release  of  energy,  the  production  of 
radioactive  atomic  species  and  the  possibility  of  a  neutron  chain 
reaction.  The  energy  release  might  reveal  itself  in  kinetic  energy 
of  the  fission  fragments  and  in  the  subsequent  radioactive  dis- 

Introduction  27 

integration  of  the  products.  The  possibility  of  a  neutron  chain 
reaction  depended  on  whether  neutrons  were  in  fact  emitted — 
a  possibility  which  required  investigation. 

1.56.  These  were  the  problems  suggested  by  the  discovery  of 
fission,  the  kind  of  problem  reported  in  the  journals  in  1939  and 
1940  and  since  then  investigated  largely  in  secret.  The  study  of 
the  fission  process  itself,  including  production  of  neutrons  and 
fast  fragments,  has  been  largely  carried  out  by  physicists  using 
counters,  cloud  chambers,  etc.  The  study  and  identification  of 
the  fission  products  has  been  carried  out  largely  by  chemists, 
who  have  had  to  perform  chemical  separations  rapidly  even  with 
submicroscopic   quantities   of  material   and  to  make  repeated 
determinations  of  the  half-lives  of  unstable  isotopes.  We  shall 
summarize  the  state  of  knowledge  as  of  June  1 940.  By  that  time 
the  principal  facts  about  fission  had  been  discovered  and  revealed 
to  the  scientific  world.  A  chain  reaction  had  not  been  obtained, 
but  its  possibility — at  least  in  principle — was  clear  and  several 
paths  that  might  lead  to  it  had  been  suggested. 



1.57.  All  the  following  information  was  generally  known  in 
June  1940,  both  here  and  abroad: 

(1)  That  three  elements — uranium,  thorium,  and  protoactinium 
— when  bombarded  by  neutrons  sometimes  split  into  approxi- 
mately equal  fragments,  and  that  these  fragments  were  isotopes 
of  elements  in  the  middle  of  the  periodic  table,  ranging  from 
selenium  (Z  =  34)  to  lanthanum  (Z  =  57). 

(2)  That  most  of  these  fission  fragments  were  unstable,  decaying 
radioactively  by  successive  emission  of  beta  particles  through  a 
series  of  elements  to  various  stable  forms. 

(3)  That  these  fission  fragments  had  very  great  kinetic  energy. 

(4)  That  fission  of  thorium  and  protoactinum  was  caused  only 

28  Introduction 

by  fast  neutrons  (velocities  of  the  order  of  thousands  of  miles 
per  second). 

(5)  That  fission  in  uranium  could  be  produced  by  fast  or  slow 
(so-called  thermal  velocity)  neutrons;  specifically,  that  thermal 
neutrons  caused  fission  in  one  isotope,  U-235,  but  not  in  the 
other,  U-238,  and  that  fast  neutrons  had  a  lower  probability  of 
causing  fission  in  U-235  than  thermal  neutrons. 

(6)  That  at  certain  neutron  speeds  there  was  a  large  capture 
cross  section  in  U-238  producing  U-239  but  not  fission. 

(7)  That  the  energy  released  per  fission  of  a  uranium  nucleus 
was  approximately  200  million  electron  volts. 

(8)  That  high-speed  neutrons  were  emitted  in  the  process  of 

(9)  That  the  average  number  of  neutrons  released  per  fission 
was  somewhere  between  one  and  three. 

(10)  That  high-speed  neutrons  could  lose  energy  by  inelastic 
collision   with   uranium   nuclei   without   any   nuclear   reaction 
taking  place. 

(11)  That  most  of  this  information  was  consistent  with  the  semi- 
empirical  theory  of  nuclear  structure  worked  out  by  Bohr  and 
Wheeler  and  others;  this  suggested  that  predictions  based  on 
this  theory  had  a  fair  chance  of  success. 


1.58.  It  was  realized  that  radiative  capture  of  neutrons  by 
U-238  would  probably  lead  by  two  successive  beta-ray  emissions 
to  the  formation  of  a  nucleus  for  which  Z  =  94  and  A  =  239. 
Consideration  of  the  Bohr- Wheeler  theory  of  fission  and  of  certain 
empirical  relations  among  the  nuclei  by  L.  A.  Turner  and  others 
suggested  that  this  nucleus  would  be  a  fairly  stable  alpha  emitter 
and  would  probably  undergo  fission  when  bombarded  by  thermal 
neutrons.  Later  the  importance  of  such  thermal  fission  to  the 
maintenance  of  the  chain  reaction  was  foreshadowed  in  private 
correspondence  and  discussion.  In  terms  of  our  present  knowledge 
and  notation  the  particular  reaction  suggested  is  as  follows: 

Introduction  29 

where  Np  and  Pu  are  the  chemical  symbols  now  used  for  the 
two  new  elements,  neptunium  and  plutonium;  on1  represents  the 
neutron,  and  _ie°  represents  an  ordinary  (negative)  electron. 
Plutonium  239  is  the  nucleus  rightly  guessed  to  be  fissionable  by 
thermal  neutrons.  It  will  be  discussed  fully  in  later  chapters. 


1.59.  By  1940  nuclear  reactions  had  been  intensively  studied 
for  over  ten  years.  Several  books  and  review  articles  on  nuclear 
physics  had  been  published.  New  techniques  had  been  developed 
for  producing  and  controlling  nuclear  projectiles,  for  studying 
artificial  radioactivity,  and  for  separating  submicroscopic  quan- 
tities of  chemical  elements  produced  by  nuclear  reactions.  Isotope 
masses  had   been   measured   accurately.   Neutron-capture  cross 
sections  had  been  measured.  Methods  of  slowing  down  neutrons 
had  been  developed.  Physiological  effects  of  neutrons  had  been 
observed;  they  had  even  been  tried  in  the  treatment  of  cancer. 
All  such  information  was  generally  available;  but  it  was  very 
incomplete.   There   were   many   gaps   and   many  inaccuracies. 
The  techniques  were  difficult  and  the  quantities  of  materials 
available  were  often  submicroscopic.  Although  the  fundamental 
principles  were  clear,  the  theory  was  full  of  unverified  assump- 
tions and  calculations  were  hard  to  make.  Predictions  made  in 
1940  by  different  physicists  of  equally  high  ability  were  often 
at  variance.  The  subject  was  in  all  too  many  respects  an  art, 
rather  than  a  science. 


1.60.  Looking  back  on  the  year  1940,  we  see  that  all  the  pre- 
requisites to  a  serious  attack  on  the  problem  of  producing  atomic 
bombs  and  controlling  atomic  power  were  at  hand.     It  had  been 
proved  that  mass  and  energy  were  equivalent.  It  had  been  proved 
that  the  neutrons  initiating  fission  of  uranium  reproduced  them- 

30  Introduction 

selves  in  the  process  and  that  therefore  a  multiplying  chain 
reaction  might  occur  with  explosive  force.  To  be  sure,  no  one 
knew  whether  the  required  conditions  could  be  achieved,  but 
many  scientists  had  clear  ideas  as  to  the  problems  involved  and 
the  directions  in  which  solutions  might  be  sought.  The  next 
chapter  of  this  report  gives  a  statement  of  the  problems  and 
serves  as  a  guide  to  the  developments  of  the  past  five  years. 



2.1.  From  the  time  of  the  first  discovery  of  the  large  amounts 
of  energy  released  in  nuclear  reactions  to  the  time  of  the  discovery 
of  uranium  fission,  the  idea  of  atomic  power  or  even  atomic  bombs 
was  discussed  off  and  on  in  scientific  circles.  The  discovery  of 
fission  made  this  talk  seem  much  less  speculative,  but  realization 
of  atomic  power  still  seemed  in  the  distant  future  and  there  was 
an  instinctive  feeling  among  many  scientists  that  it  might  not, 
in  fact,  ever  be  realized.  During  1939  and  1940  many  public 
statements,  some  of  them  by  responsible  scientists,  called  atten- 
tion to  the  enormous  energy  available  in  uranium  for  explosives 
and  for  controlled  power,  so  that  U-235  became  a  familiar  by- 
word indicating  great  things  to  come.   The  possible  military 
importance  of  uranium  fission  was  called  to  the  attention  of  the 
government  (see  Chapter  III),  and  in  a  conference  with  repre- 
sentatives of  the  Navy  Department  in  March  1939  Fermi  sug- 
gested the  possibility  of  achieving  a  controllable  reaction  using 
slow  neutrons  or  a  reaction  of  an  explosive  character  using  fast 
neutrons.  He  pointed  out,  however,  that  the  data  then  available 
might  be  insufficient  for  accurate  predictions. 

2.2.  By  the  summer  of  1940  it  was  possible  to  formulate  the 
problem  fairly  clearly,  although  it  was  still  far  from  possible  to 
answer  the  various  questions  involved  or  even  to  decide  whether 
a  chain  reaction  ever  could  be  obtained.  In  this  chapter  we  shall 
give  a  statement  of  the  problem  in  its  entirety.  For  purposes  of 
clarification  we  may  make  use  of  some  knowledge  which  actually 
was  not  acquired  until  a  later  date. 


2.3.  The  principle  of  operation  of  an  atomic  bomb  or  power 
plant  utilizing  uranium  fission  is  simple  enough.  If  one  neutron 


32  Statement  of  the  Problem 

causes  a  fission  that  produces  more  than  one  new  neutron,  the 
number  of  fissions  may  increase  tremendously  with  the  release 
of  enormous  amounts  of  energy.  It  is  a  question  of  probabilities. 
Neutrons  produced  in  the  fission  process  may  escape  entirely 
from  the  uranium,  may  be  captured  by  uranium  in  a  process  not 
resulting  in  fission,  or  may  be  captured  by  an  impurity.  Thus  the 
question  of  whether  a  chain  reaction  does  or  does  not  go  depends 
on  the  result  of  a  competition  among  four  processes: 

(1)  escape, 

(2)  non-fission  capture  by  uranium, 

(3)  non-fission  capture  by  impurities, 

(4)  fission  capture. 

If  the  loss  of  neutrons  by  the  first  three  processes  is  less  than  the 
surplus  produced  by  the  fourth,  the  chain  reaction  occurs;  other- 
wise it  does  not.  Evidently  any  one  of  the  first  three  processes 
may  have  such  a  high  probability  in  a  given  arrangement  that 
the  extra  neutrons  created  by  fission  will  be  insufficient  to  keep 
the  reaction  going.  For  example,  should  it  turn  out  that  process 
(2) — non-fission  capture  by  uranium — has  a  much  higher  prob- 
ability than  fission  capture,  there  would  presumably  be  no 
possibility  of  achieving  a  chain  reaction. 

2.4.  An  additional  complication  is  that  natural  uranium  con- 
tains three  isotopes:  U-234,  U-235,  and  U-238,  present  to  the 
extent  of  approximately  0.006,  0.7,  and  99.3  percent,  respectively. 
We  have  already  seen  that  the  probabilities  of  processes  (2) 
and  (4)  are  different  for  different  isotopes.  We  have  also  seen 
that   the   probabilities   are    different   for   neutrons   of  different 

2.5.  We  shall  now  consider  the  limitations  imposed  by  the 
first  three  processes  and  how  their  effects  can  be  minimized. 


2.6.  The  relative  number  of  neutrons  which  escape  from  a 
quantity  of  uranium  can  be  minimized  by  changing  the  size 

Statement  of  the  Problem  33 

and  shape.  In  a  sphere  any  surface  effect  is  proportional  to  the 
square  of  the  radius,  and  any  volume  effect  is  proportional  to  the 
cube  of  the  radius.  Now  the  escape  of  neutrons  from  a  quantity 
of  uranium  is  a  surface  effect  depending  on  the  area  of  the  surface, 
but  fission  capture  occurs  throughout  the  material  and  is  there- 
fore a  volume  effect.  Consequently  the  greater  the  amount  of 
uranium,  the  less  probable  it  is  that  neutron  escape  will  pre- 
dominate over  fission  capture  and  prevent  a  chain  reaction. 
Loss  of  neutrons  by  non-fission  capture  is  a  volume  effect  like 
neutron  production  by  fission  capture,  so  that  increase  in  size 
makes  no  change  in  its  relative  importance. 

2.7.  The  critical  size  of  a  device  containing  uranium  is  defined 
as  the  size  for  which  the  production  of  free  neutrons  by  fission 
is  just  equal  to  their  loss  by  escape  and  by  non-fission  capture. 
In  other  words,  if  the  size  is  smaller  than  critical,  then — by 
definition — no  chain  reaction  will  sustain  itself.  In  principle  it 
was  possible  in  1940  to  calculate  the  critical  size,  but  in  practice 
the  uncertainty  of  the  constants  involved  was  so  great  that  the 
various  estimates  differed  widely.  It  seemed  not  improbable  that 
the  critical  size  might  be  too  large  for  practical  purposes.  Even 
now  estimates  for  untried  arrangements  vary  somewhat  from 
time  to  time  as  new  information  becomes  available. 


2.8.  In  Chapter  I  we  said  that  thermal  neutrons  have  the 
highest  probability  of  producing  fission  of  U-235  but  we  also 
said  that  the  neutrons  emitted  in  the  process  of  fission  had  high 
speeds.  Evidently  it  was  an  oversimplification  to  say  that  the 
chain  reaction  might  maintain  itself  if  more  neutrons  were 
created  by  fission  than  were  absorbed.  For  the  probability  both 
of  fission  capture  and  of  non-fission  capture  depends  on  the  speed 
of  the  neutrons.  Unfortunately,  the  speed  at  which  non-fission 
capture  is  most  probable  is  intermediate  between  the  average 
speed  of  neutrons  emitted  in  the  fission  process  and  the  speed 
at  which  fission  capture  is  most  probable. 

34  Statement  of  the  Problem 

2.9.  For  some  years  before  the  discovery  of  fission,  the  cus- 
tomary way  of  slowing  down  neutrons  was  to  cause  them  to 
pass  through  material  of  low  atomic  weight,  such  as  hydrogenous 
material.  The  process  of  slowing  down  or  moderation  is  simply 
one  of  elastic  collisions  between  high-speed  particles  and  particles 
practically    at   rest.    The  more  nearly  identical  the  masses  of 
neutron  and  struck  particle  the  greater  the  loss  of  kinetic  energy 
by  the  neutron.  Therefore  the  light  elements  are  most  effective 
as  "moderators,"  i.e.,  slowing  down  agents,  for  neutrons. 

2.10.  It  occurred  to  a  number  of  physicists  that  it  might  be 
possible  to  mix  uranium  with  a  moderator  in  such  a  way  that 
the  high-speed  fission  neutrons,  after  being  ejected  from  uranium 
and  before  re-encountering  uranium  nuclei,  would  have  their 
speeds  reduced  below  the  speeds  for  which  non-fission  capture 
is  highly  probable.  Evidently  the  characteristics  of  a  good  moder- 
ator are  that  it  should  be  of  low  atomic  weight  and  that  it  should 
have  little  or  no  tendency  to  absorb  neutrons.  Lithium  and  boron 
are  excluded  on  the  latter  count.  Helium  is  difficult  to  use  because 
it  is  a  gas  and  forms  no  compounds.  The  choice  of  moderator 
therefore    lay    among    hydrogen,    deuterium,    beryllium,    and 
carbon.  Even  now  no  one  of  these  substances  can  be  excluded 
from  the  list  of  practical  possibilities.  It  was  E.  Fermi  and  L. 
Szilard  who  proposed  the  use  of  graphite  as  a  moderator  for  a 
chain  reaction. 


2.11.  The  general  scheme  of  using  a  moderator  mixed  with 
the  uranium  was  pretty  obvious.  A  specific  manner  of  using  a 
moderator  was  first  suggested  in  this  country,  so  far  as  we  can 
discover,  by  Fermi  and  Szilard.  The  idea  was  to  use  lumps  of 
uranium  of  considerable  size  imbedded  in  a  matrix  of  moderator 
material.  Such  a  lattice  can  be  shown  to  have  real  advantages 
over  a  homogeneous  mixture.  As  the  constants  were  more  accu- 
rately determined,  it  became  possible  to  calculate  theoretically 
the  type  of  lattice  that  would  be  most  effective. 



















/  PA 

ST\                       < 




|          MODERATOR         ] 

/    SLOW   NE 

UTRONS     \ 





/    I    \ 

AND  SO   ON. 



Statement  of  the  Problem  35 


2.12.  In  Chapter  I  it  was  stated  that  for  neutrons  of  certain 
intermediate  speeds  (corresponding  to  energies  of  a  few  electron 
volts)  U-238  has  a  large  capture  cross  section  for  the  production 
of  U-239  but  not  for  fission.  There  is  also  a  considerable  prob- 
ability of  inelastic  (i.e.,  non-capture-producing)  collisions  between 
high-speed  neutrons  and  U-238  nuclei.  Thus  the  presence  of  the 
U-238  tends  both  to  reduce  the  speed  of  the  fast  neutrons  and  to 
effect  the  capture  of  those  of  moderate  speed.  Although  there 
may  be  some  non-fission  capture  by  U-235,  it  is  evident  that 
if  we  can  separate  the  U-235  from  the  U-238  and  discard  the 
U-238,  we  can  reduce  non-fission  capture  and  can  thus  promote 
the  chain  reaction.  In  fact,  the  probability  of  fission  of  U-235 
by  high-speed  neutrons  may  be  great  enough  to  make  the  use 
of  a  moderator  unnecessary  once  the  U-238  has  been  removed. 
Unfortunately,  U-235  is  present  in  natural  uranium  only  to  the 
extent    of   about    one    part  in   140.   Also,   the  relatively  small 
difference  in  mass  between  the  two  isotopes  makes  separation 
difficult.  In  fact,  in  1940  no  large-scale  separation  of  isotopes 
had  ever  been  achieved  except  for  hydrogen,  whose  two  isotopes 
differ  in  mass  by  a  factor  of  two.  Nevertheless,  the  possibility  of 
separating  U-235  was  recognized  early  as  being  of  the  greatest 
importance,  and  such  separation  has,  in  fact,  been  one  of  the 
two  major  lines  of  Project  effort  during  the  past  five  years. 


2.13.  It  has  been  stated  above  that  the  cross  section  for  capture 
of  neutrons  varies  greatly  among  different  materials.  In  some  it 
is  very  high  compared  to  the  maximum  fission  cross  section  of 
uranium.  If,  then,  we  are  to  hope  to  achieve  a  chain  reaction, 
we  must  reduce  effect  (3) — non-fission  capture  by  impurities — 
to  the  point  where  it  is  not  serious.  This  means  very  careful 
purification  of  the  uranium  metal  and  very  careful  purification 
of  the  moderator.   Calculations  show  that  the  maximum  per- 

36  Statement  of  the  Problem 

missible  concentrations  of  many  impurity  elements  are  a  few 
parts  per  million — in  either  the  uranium  or  the  moderator.  When 
it  is  recalled  that  up  to  1 940  the  total  amount  of  uranium  metal 
produced  in  this  country  was  not  more  than  a  few  grams  and 
even  this  was  of  doubtful  purity,  that  the  total  amount  of  metallic 
beryllium  produced  in  this  country  was  not  more  than  a  few 
pounds,  that  the  total  amount  of  concentrated  deuterium  pro- 
duced was  not  more  than  a  few  pounds,  and  that  carbon  had 
never  been  produced  in  quantity  with  anything  like  the  purity 
required  of  a  moderator,  it  is  clear  that  the  problem  of  producing 
and  purifying  materials  was  a  major  one. 


2.14.  The  problems  that  have  been  discussed  so  far  have  to 
do  merely  with  the  realization  of  the  chain  reaction.  If  such  a 
reaction  is  going  to  be  of  use,  we  must  be  able  to  control  it.  The 
problem  of  control  is  different  depending  on  whether  we  are 
interested  in  steady  production  of  power  or  in  an  explosion.  In 
general,  the  steady  production  of  atomic  power  requires  a  slow- 
neutron-induced  fission  chain  reaction  occurring  in  a  mixture 
or  lattice  of  uranium  and  moderator,  while  an  atomic  bomb 
requires  a  fast-neutron-induced  fission  chain  reaction  in  U-235 
or  Pu-239,  although  both  slow-  and  fast-neutron  fission  may  con- 
tribute in  each  case.  It  seemed  likely,  even  in  1940,  that  by  using 
neutron  absorbers  a  power  chain  reaction  could  be  controlled. 
It  was  also  considered  likely,  though  not  certain,  that  such  a  chain 
reaction  would  be  self-limiting  by  virtue  of  the  lower  probability 
of  fission-producing  capture  when  a  higher  temperature  was 
reached.  Nevertheless,  there  was  a  possibility  that  a  chain-reacting 
system  might  get  out  of  control,  and  it  therefore  seemed  necessary 
to  perform  the  chain-reaction  experiment  in  an  uninhabited 


2.15.  Up  to  this  point  we  have  been  discussing  how  to  produce 
and  control  a  nuclear  chain  reaction  but  not  how  to  make  use 

Statement  of  the  Problem  37 

of  it.  The  technological  gap  between  producing  a  controlled 
chain  reaction  and  using  it  as  a  large-scale  power  source  or  an 
explosive  is  comparable  to  the  gap  between  the  discovery  of  fire 
and  the  manufacture  of  a  steam  locomotive. 

2.16.  Although    production    of   power    has    never   been    the 
principal  object  of  this  project,  enough  attention  has  been  given 
to  the  matter  to  reveal  the  major  difficulty:  the  attainment  of 
high- temperature  operation.  An  effective  heat  engine  must  not 
only  develop  heat  but  must  develop  heat  at  a  high  temperature. 
To  run  a  chain-reacting  system  at  a  high  temperature  and  to 
convert  the  heat  generated  to  useful  work  is  very  much  more 
difficult  than  to  run  a  chain-reacting  system  at  a  low  temperature. 

2.17.  Of  course,  the  proof  that  a  chain  reaction  is  possible  does 
not  itself  insure  that  nuclear  energy  can  be  effective  in  a  bomb. 
To  have  an  effective  explosion  it  is  necessary  that  the  chain 
reaction  build  up  extremely  rapidly;   otherwise  only  a  small 
amount  of  the  nuclear  energy  will  be  utilized  before  the  bomb 
flies  apart  and  the  reaction  stops.  It  is  also  necessary  that  no 
premature  explosion  occur.  This  entire  "detonation"  problem 
was  and  still  remains  one  of  the  most  difficult  problems  in  de- 
signing a  high-efficiency  atomic  bomb. 


2.18.  So  far,  all  our  discussion  has  been  primarily  concerned 
with  the  use  of  uranium  itself.  We  have  already  mentioned  the 
suggestion  that  the  element  of  atomic  number  94  and  mass  239, 
commonly  referred  to  as  plutonium,  might  be  very  effective. 
Actually,  we  now  believe  it  to  be  of  value  comparable  to  pure 
U-235.  We  have  mentioned  the  difficulty  of  separating  U-235 
from  the  more  abundant  isotope  U-238.  These  two  isotopes  are, 
of  course,  chemically  identical.  But  plutonium,  although  pro- 
duced from  U-238,  is  a  different  chemical  element.  Therefore, 
if  a  process  could  be  worked  out  for  converting  some  of  the  U-238 
to  plutonium,  a  chemical  separation  of  the  plutonium  from  ura- 
nium might  prove  more  practicable  than  the  isotopic  separation 
of  U-235  from  U-238. 

38  Statement  of  the  Problem 

2.19.  Suppose  that  we  have  set  up  a  controllable  chain  reac- 
tion in  a  lattice  of  natural  uranium  and  a  moderator — say  carbon, 
in  the  form  of  graphite.  Then  as  the  chain  reaction  proceeds, 
neutrons  are  emitted  in  the  process  of  fission  of  the  U-235  and 
many  of  these  neutrons  are  absorbed  by  U-238.  This  produces 
U-239,  each  atom  of  which  then  emits  a  beta  particle,  becoming 
neptunium  (osNp239).  Neptunium,  in  turn,  emits  another  beta 
particle,  becoming  plutonium  (94Pu239),  which  emits  an  alpha 
particle,  decaying  again  to  U-235,  but  so  slowly  that  in  effect 
it  is  a  stable  element.  (See  figure  on  p.  8.)  If,  after  the  reaction 
has  been  allowed  to  proceed  for  a  considerable  time,  the  mixture 
of  metals  is  removed,  it  may  be  possible  to  extract  the  plutonium 
by   chemical   methods  and  purify  it  for  use  in  a  subsequent 
fission  chain  reaction  of  an  explosive  nature. 


2.20.  Three   ways   of  increasing   the   likelihood   of  a   chain 
reaction  have  been  mentioned:  use  of  a  moderator;  attainment 
of  high  purity  of  materials;  use  of  special  material,  either  U-235 
or  Pu.  The  three  procedures  are  not  mutually  exclusive,  and 
many  schemes  have  been  proposed  for  using  small  amounts  of 
separated  U-235  or  Pu-239  in  a  lattice  composed  primarily  of 
ordinary  uranium  or  uranium  oxide  and  of  a  moderator  or  two 
different  moderators.  Such  proposed  arrangements  are  usually 
called  "enriched  piles." 


2.21.  All  our  previous  discussion  has  centered  on  the  direct  or 
indirect  use  of  uranium,  but  it  was  known  that  both  thorium 
and  protoactinium  also  underwent  fission  when  bombarded  by 
high-speed  neutrons.  The  great  advantage  of  uranium,  at  least 
for  preliminary  work,  was  its  susceptibility  to  slow  neutrons. 
There  was  not  very  much  consideration  given  to  the  other  two 
substances.    Protoactinium    can    be   eliminated    because   of   its 

Statement  of  the  Problem  39 

scarcity  in  nature.  Thorium  is  relatively  plentiful  but  has  no 
apparent  advantage  over  uranium. 

2.22.  It  is  not  to  be  forgotten  that  theoretically  many  nuclear 
reactions  might  be  used  to  release  energy.  At  present  we  see  no 
way    of   initiating    or    controlling    reactions    other    than    those 
involving  fission,  but  some  such  synthesis  as  has  already  been 
mentioned  as  a  source  of  solar  energy  may  eventually  be  pro- 
duced in  the  laboratory. 


2.23.  Obviously  it  was  impossible  in  the  summer  of  1940  to 
make  more  than  guesses  as  to  what  amounts  of  materials  would 
be  needed  to  produce: 

(1)  a  chain  reaction  with  use  of  a  moderator: 

(2)  a  chain-reaction  bomb  in  pure,  or  at  least  enriched,  U-235 
or  plutonium. 

A  figure  of  one  to  one  hundred  kilograms  of  U-235  was  com- 
monly given  at  this  time  for  the  critical  size  of  a  bomb.  This 
would,  of  course,  have  to  be  separated  from  at  least  140  times 
as  much  natural  uranium.  For  a  slow-neutron  chain  reaction 
using  a  moderator  and  unseparated  uranium  it  was  almost 
certain  that  tons  of  metal  and  of  moderator  would  be  required. 


2.24.  Estimates  of  the  composition  of  the  earth's  crust  show 
uranium  and  thorium  both  present  in  considerable  quantities 
(about  4  parts  per  million  of  uranium  and  12  parts  per  million 
of  thorium  in  the  earth's  crust).  Deposits  of  uranium  ore  are 
known  to  exist  in  Colorado,  in  the  Great  Bear  Lake  region  of 
northern  Canada,  in  Joachimstal  in  Czechoslovakia,  and  in  the 
Belgian  Congo.  Many  other  deposits  of  uranium  ore  are  known, 
but  their  extent  is  in  many  cases  unexplored.  Uranium  is  always 
found  with  radium  although  in  much  larger  quantity.  Both  are 
often  found  with  vanadium  ores.  Small  quantities  of  uranium 
oxide  have  been  used  for  many  years  in  the  ceramics  industry. 

2.25.  Thorium  is  also  rather  widely  distributed,  occurring  as 

40  Statement  of  the  Problem 

thorium  oxide  in  fairly  high  concentration  in  monazite  sands. 
Such  sands  are  found  to  some  extent  in  this  country  but  particu- 
larly in  Brazil  and  in  British  India. 

2.26.  Early  rough  estimates,  which  are  probably  optimistic, 
were  that  the  nuclear  energy  available  in  known  deposits  of 
uranium  was  adequate  to  supply  the  total  power  needs  of  this 
country  for  200  years  (assuming  utilization  of  U-238  as  well  as 

2.27.  As  has  already  been  mentioned,  little  or  no  uranium 
metal  had  been  produced  up  to  1940  and  information  was  so 
scant  that  even  the  melting  point  was  not  known.  (For  example, 
the  Handbook  of  Physics  and  Chemistry  for  1 943-1 944  says  only  that 
the  melting  point  is  below  1850°  C.  whereas  we  now  know  it 
to  be  in  the  neighborhood  of  1150°.)  Evidently,  as  far  as  uranium 
was  concerned,  there  was  no  insurmountable  difficulty  as  regards 
obtaining  raw  materials  or  producing  the  metal,  but  there  were 
very  grave  questions  as  to  how  long  it  would  take  and  how  much 
it  would  cost  to  produce  the  necessary  quantities  of  pure  metal. 

2.28.  Of  the  materials  mentioned  above  as  being  suitable  for 
moderators,  deuterium  had  the  most  obvious  advantages.  It  is 
present  in  ordinary  hydrogen  to  the  extent  of  about  one  part  in 
5,000.  By  1940  a  number  of  different  methods  for  separating  it 
from  hydrogen  had  been  developed,  and  a  few  liters  had  been 
produced  in  this  country  for  experimental  purposes.  The  only 
large-scale  production  had  been  in  a  Norwegian  plant,  from 
which  several  hundred  liters  of  heavy  water  (DzO,  deuterium 
oxide)  had  come.  As  in  the  case  of  uranium,  the  problem  was 
one  of  cost  and  time. 

2.29.  Beryllium  in  the  form  of  beryllium  silicates  is  widely 
found  but  only  in  small  quantities  of  ore.  Its  use  as  an  alloying 
agent  has  become  general  in  the  last  few  years;  for  such  use, 
however,  it  is  not  necessary  to  produce  the  beryllium  in  metallic 
form.  In  1940  only  700  pounds  of  the  metal  were  produced  in 
this  country. 

2.30.  As   far   as   carbon   was   concerned,    the   situation   was 
obviously  quite  different.  There  were  many  hundreds  of  tons 

Statement  of  the  Problem  41 

of  graphite  produced  every  year  in  this  country.  This  was  one 
of  the  reasons  why  graphite  looked  very  desirable  as  a  moderator. 
The  difficulties  lay  in  obtaining  sufficient  quantities  of  graphite 
of  the  required  purity,  particularly  in  view  of  the  expanding 
needs  of  war  industry. 


2.31.  Requirements  of  time  and  money  depended  not  only 
on  many  unknown  scientific  and  technological  factors  but  also 
on  policy  decisions.  Evidently  years  of  time  and  millions  of  dollars 
might  be  required  to  achieve  the  ultimate  objective.  About  all 
that  was  attempted  at  this  time  was  the  making  of  estimates  as 
to  how  long  it  would  take  and  how  much  it  would  cost  to  clarify 
the  scientific  and  technological  prospects.  It  looked  as  if  it  would 
not  be  a  very  great  undertaking  to  carry  along  the  development 
of  the  thermal-neutron  chain  reaction  in  a  graphite-uranium 
lattice  to  the  point  of  finding  out  whether  the  reaction  would 
in  fact  go.  Estimates  made  at  the  time  were  that  approximately 
a  year  and  $100,000  would  be  required  to  get  an  answer.  These 
estimates  applied  to  a  chain-reacting  system  of  very  low  power 
without  a  cooling  system  or  any  means  for  using  the  energy 


2.32.  It  had  been  known  for  a  long  time  that  radioactive 
materials  were  dangerous.  They  give  off  very  penetrating  radia- 
tions— gamma  rays — which  are  much  like  X-rays  in  their  phy- 
siological effects.  They  also  give  off  beta  and  alpha  rays  which, 
although  less  penetrating,  can  still  be  dangerous.  The  amounts 
of  radium  used  in  hospitals  and  in  ordinary  physical  measure- 
ments usually  comprise  but  a  few  milligrams.  The  amounts  of 
radioactive  material  produced  by  the  fission  of  uranium  in  a 
relatively  small   chain-reacting   system   may   be   equivalent   to 
hundreds  or  thousands  of  grams  of  radium.  A  chain-reacting 
system   also   gives  off  intense   neutron  radiation  known   to   be 

42  Statement  of  the  Problem 

comparable  to  gamma  rays  as  regards  health  hazards.  Quite 
apart  from  its  radioactive  properties,  uranium  is  poisonous 
chemically.  Thus,  nearly  all  work  in  this  field  is  hazardous — 
particularly  work  on  chain  reactions  and  the  resulting  radio- 
active products. 


2.33.  There  were  two  ways  of  attacking  the  problem.  One  was 
to  conduct  elaborate  series  of  accurate  physical  measurements  on 
absorption  cross  sections  of  various  materials  for  various  neutron- 
induced  processes  and  various  neutron  energies.  Once  such  data 
were  available,  calculations  as  to  what  might  be  done  in  the 
way  of  a  chain  reaction  could  be  made  with  fair  accuracy.  The 
other  approach  was  the  purely  empirical  one  of  mixing  uranium 
or  uranium  compounds  in  various  ways  with  various  moderators 
and  observing  what  happened.  Similar  extremes  of  method  were 
possible  in  the  case  of  the  isotope-separation  problem.  Actually 
an  intermediate  or  compromise  approach  was  adopted  in  both 


2.34.  The  expected  military  advantages  of  uranium  bombs 
were  far  more  spectacular  than  those  of  a  uranium  power  plant. 
It  was  conceivable  that  a  few  uranium  bombs  might  be  decisive 
in  winning  the  war  for  the  side  first  putting  them  into  use.  Such 
thoughts  were  very  much  in  the  minds  of  those  working  in  this 
field,    but   the   attainment   of  a   slow-neutron    chain   reaction 
seemed  a  necessary  preliminary  step  in  the  development  of  our 
knowledge  and  became  the  first  objective  of  the  group  interested 
in  the  problem.  This  also  seemed  an  important  step  in  con- 
vincing military  authorities  and   the  more  skeptical  scientists 
that  the  whole  notion  was  not  a  pipe  dream.  Partly  for  these 
reasons  and  partly  because  of  the  extreme  secrecy  imposed  about 
this  time,  the  idea  of  an  atomic  bomb  does  not  appear  much  in 
the  records  between  the  summer  of  1940  and  the  fall  of  1941. 

Statement  of  the  Problem  43 


2.35.  If  all  the  atoms  in  a  kilogram  of  U-235  undergo  fission, 
the  energy  released  is  equivalent  to  the  energy  released  in  the 
explosion  of  about  20,000  short  tons  of  TNT.  If  the  critical  size 
of  a  bomb  turns  out  to  be  practical — say,  in  the  range  of  one  to 
one  hundred  kilograms — and  all  the  other  problems  can   be 
solved,  there  remain  two  questions.  First,  how  large  a  percentage 
of  the  fissionable  nuclei  can  be  made  to  undergo  fission  before 
the  reaction  stops;  i.e.,  what  is  the  efficiency  of  the  explosion? 
Second,  what  is  the  effect  of  so  concentrated  a  release  of  energy? 
Even  if  only  1  per  cent  of  the  theoretically  available  energy  is 
released,  the  explosion  will  still  be  of  a  totally  different  order  of 
magnitude  from  that  produced  by  any  previously  known  type 
of  bomb.  The  value  of  such  a  bomb  was  thus  a  question  for 
military  experts  to  consider  very  carefully. 


2.36.  It  had  been  established   (1)   that  uranium  fission  did 
occur  with  release  of  great  amounts  of  energy;  and  (2)  that  in 
the  process  extra  neutrons  were  set  free  which  might  start  a 
chain  reaction.  It  was  not  contrary  to  any  known  principle  that 
such  a  reaction  should  take  place  and  that  it  should  have  very 
important  military  application  as  a  bomb.  However,  the  idea 
was  revolutionary  and   therefore  suspect;   it  was  certain   that 
many  technical  operations  of  great  difficulty  would  have  to  be 
worked  out  before  such  a  bomb  could  be  produced.  Probably 
the  only  materials  satisfactory  for  a  bomb  were  either  U-235, 
which  would  have  to  be  separated  from  the  140- times  more 
abundant  isotope  U-238,  or  Pu-239,  an  isotope  of  the  hitherto 
unknown  element  plutonium,  which  would  have  to  be  generated 
by  a  controlled  chain-reacting  process  itself  hitherto  unknown. 
To  achieve  such  a  controlled  chain  reaction  it  was  clear  that 
uranium  metal  and  heavy  water  or  beryllium  or  carbon  might 
have  to  be  produced  in  great  quantity  with  high  purity.  Once 
bomb  material  was  produced  a  process  would  have  to  be  devel- 

44  Statement  of  the  Problem 

oped  for  using  it  safely  and  effectively.  In  some  of  the  processes, 
health  hazards  of  a  new  kind  would  be  encountered. 


2.37.  By  the  summer  of  1940  the  National  Defense  Research 
Committee  had  been  formed  and  was  asking  many  of  the  scien- 
tists in  the  country  to  work  on  various  urgent  military  problems. 
Scientific  personnel  was  limited  although  this  was  not  fully 
realized  at  the  time.  It  was,  therefore,  really  difficult  to  decide 
at  what  rate  work  should  be  carried  forward  on  an  atomic 
bomb.  The  decision  had  to  be  reviewed  at  frequent  intervals 
during  the  subsequent  four  years.  An  account  of  how  these  policy 
decisions  were  made  is  given  in  Chapters  III  and  V. 



3.1.  The  announcement  of  the  hypothesis  of  fission  and  its 
experimental  confirmation  took  place  in  January  1939,  as  has 
already  been  recounted  in  Chapter  I.  There  was  immediate 
interest  in  the  possible  military  use  of  the  large  amounts  of  energy 
released  in  fission.  At  that  time  American-born  nuclear  physicists 
were  so  unaccustomed  to  the  idea  of  using  their  science  for  mili- 
tary purposes  that  they  hardly  realized  what  needed  to  be  done. 
Consequently  the  early  efforts  both  at  restricting  publication 
and  at  getting  government  support  were  stimulated  largely  by 
a  small  group  of  foreign-born  physicists  centering  on  L.  Szilard 
and  including  E.  Wigner,  E.  Teller,  V.  F.  Weisskopf,  and  E. 

S'. In  the  spring  of  1939  the  group  mentioned  above  enlisted 
Niels  Bohr's  cooperation  in  an  attempt  to  stop  publication  of 
further  data  by  voluntary  agreement.  Leading  American  and 
British  physicists  agreed,  but  F.  Joliot,  France's  foremost  nuclear 
physicist,  refused,  apparently  because  of  the  publication  of  one 
letter  in  the  Physical  Review  sent  in  before  all  Americans  had  been 
brought  into  the  agreement.  Consequently  publication  con- 
tinued freely  for  about  another  year  although  a  few  papers  were 
withheld  voluntarily  by  their  authors. 

3.3.  At  the  April  1940  meeting  of  the  Division  of  Physical 
Sciences  of  the  National  Research  Council,  G.  Breit  proposed 
formation  of  a  censorship  committee  to  control  publication  in 
all  American  scientific  journals.  Although  the  reason  for  this 


46  Administrative  History  to  1941 

suggestion  was  primarily  the  desire  to  control  publication  of 
papers  on  uranium  fission,  the  "Reference  Committee"  as  finally 
set  up  a  little  later  that  spring  (in  the  National  Research  Coun- 
cil) was  a  general  one,  and  was  organized  to  control  publication 
policy  in  all  fields  of  possible  military  interest.  The  chairman  of 
the  committee  was  L.  P.  Eisenhart;  other  members  were  G.  Breit, 
W.  M.  Clark,  H.  Fletcher,  E.  B.  Fred,  G.  B.  Pegram,  H.  C.  Urey, 
L.  H.  Weed,  and  E.  G.  Wever.  Various  subcommittees  were 
appointed,  the  first  one  of  which  had  to  do  with  uranium  fission. 
G.  Breit  served  as  chairman  of  this  subcommittee;  its  other 
members  were  J.  W.  Beams,  L.  J.  Briggs,  G.  B.  Pegram,  H.  C. 
Urey,  and  E.  Wigner.  In  general,  the  procedure  followed  was  to 
have  the  editors  of  various  journals  send  copies  of  papers  in  this 
field,  in  cases  where  the  advisability  of  publication  was  in  doubt, 
either  directly  to  Breit  or  indirectly  to  him  through  Eisenhart. 
Breit  then  usually  circulated  them  to  all  members  of  the  sub- 
committee for  consideration  as  to  whether  or  not  they  should  be 
published^  and  informed  the  editors  as  to  the  outcome.  This 
arrangement  was  very  successful  in  preventing  publication  and 
was  still  nominally  in  effect,  in  modified  form,  in  June  1945. 
Actually  the  absorption  of  most  physicists  in  this  country  into 
war  work  of  one  sort  of  another  soon  reduced  the  number  of 
papers  referred  to  the  committee  practically  to  the  vanishing 
point.  It  is  of  interest  to  note  that  this  whole  arrangement  was  a 
purely  voluntary  one;  the  scientists  of  the  country  are  to  be 
congratulated  on  their  complete  cooperation.  It  is  to  be  hoped 
that  it  will  be  possible  after  the  war  to  publish  these  papers  at 
least  in  part  so  that  their  authors  may  receive  proper  professional 
credit  for  their  contributions. 


3.4.  On  the  positive  side — government  interest  and  support  of 
research  in  nuclear  physics — the  history  is  a  much  more  com- 
plicated one.  The  first  contact  with  the  government  was  made  by 
Pegram  of  Columbia  in  March  1939.  Pegram  telephoned  to  the 

Administrative  History  to  1941  47 

Navy  Department  and  arranged  for  a  conference  between 
representatives  of  the  Navy  Department  and  Fermi.  The  only 
outcome  of  this  conference  was  that  the  Navy  expressed  interest 
and  asked  to  be  kept  informed.  The  next  attempt  to  interest  the 
government  was  stimulated  by  Szilard  and  Wigner.  In  July 
1939  they  conferred  with  A.  Einstein,  and  a  little  later  Einstein, 
Wigner,  and  Sziiard  discussed  the  problem  with  Alexander 
Sachs  of  New  York.  In  the  fall  Sachs,  supported  by  a  letter  from 
Einstein,  explained  to  President  Roosevelt  the  desirability  of 
encouraging  work  in  this  field.  The  President  appointed  a  com- 
mittee, known  as  the  "Advisory  Committee  on  Uranium"  and 
consisting  of  Briggs  (director  of  the  Bureau  of  Standards)  as  chair- 
man, Colonel  K.  F.  Adamson  of  the  Army  Ordnance  Department, 
and  Commander  G.  C.  Hoover  of  the  Navy  Bureau  of  Ordnance, 
and  requested  this  committee  to  look  into  the  problem.  This  was 
the  only  committee  on  uranium  that  had  official  status  up  to  the 
time  of  organization  of  the  National  Defense  Research  Committee 
in  June  1940.  The  committee  met  very  informally  and  included 
various  additional  scientific  representatives  in  its  meetings. 

3.5.  The  first  meeting  of  the  Uranium  Committee  was  on 
October  21,  1939  and  included,  besides  the  committee  members, 
F.  L.  Mohler,  Alexander  Sachs,  L.  Szilard,  E.  Wigner,  E.  Teller, 
and  R.  B.  Roberts.  The  result  of  this  meeting  was  a  report  dated 
November  1,  1939,  and  transmitted  to  President  Roosevelt  by 
Briggs,  Adamson,  and  Hoover.  This  report  made  eight  recom- 
mendations,  which  need  not  be  enumerated   in   detail.   It  is 
interesting,  however,  that  it  specifically  mentions  both  atomic 
power  and  an  atomic  bomb  as  possibilities.  It  specifically  recom- 
mended procurement  of  4  tons  of  graphite  and  50  tons  of  uranium 
oxide  for  measurements  of  the  absorption  cross  section  of  carbon. 
Others  of  the  recommendations  either  were  of  a  general  nature 
or  were  never  carried  out.  Apparently  a  memorandum  prepared 
by  Szilard  was  more  or  less  the  basis  of  the  discussion  at  this 

3.6.  The  first  transfer  of  funds  ($6,000)  from  the  Army  and 
Navy  to  purchase  materials  in  accordance  with  the  recommenda- 

48  Administrative  History  to  1941 

tion  of  November  1  is  reported  in  a  memorandum  from  Briggs 
to  General  E.  M.  Watson  (President  Roosevelt's  aide)  on 
February  20,  1940.  The  next  meeting  of  the  "Advisory  Com- 
mittee on  Uranium"  was  on  April  28,  1940  and  was  attended  by 
Sachs,  Wigner,  Pegram,  Fermi,  Szilard,  Briggs,  Admiral  H.  G. 
Bowen,  Colonel  Adamson,  and  Commander  Hoover.  By  the 
time  of  this  meeting  two  important  new  factors  had  come  into 
the  picture.  First,  it  had  been  discovered  that  the  uranium  fission 
caused  by  neutrons  of  thermal  velocities  occurred  in  the  U-235 
isotope  only.  Second,  it  had  been  reported  that  a  large  section 
of  the  Kaiser  Wilhelm  Institute  in  Berlin  had  been  set  aside  for 
research  on  uranium.  Although  the  general  tenor  of  the  discus- 
sion at  this  meeting  seems  to  have  been  that  the  work  should  be 
pushed  more  vigorously,  no  definite  recommendations  were 
made.  It  was  pointed  out  that  the  critical  measurements  on 
carbon  already  under  way  at  Columbia  should  soon  give  a  result, 
and  the  implication  was  that  definite  recommendations  should 
wait  for  such  a  result. 

3.7.  Within  the  next  few  weeks  a  number  of  people  concerned, 
particularly  Sachs,  urged  the  importance  of  greater  support  and 
of  better  organization.   Their  hand  was  strengthened   by   the 
Columbia  results  (as  reported,  for  example,  in  a  letter  from  Sachs 
to  General  Watson  on  May  15,  1940)  showing  that  the  carbon 
absorption  was   appreciably  lower  than   had   been   previously 
thought  and  that  the  probability  of  carbon  being  satisfactory 
as   a   moderator   was   therefore   considerable.    Sachs   was   also 
active  in  looking  into  the  question  of  ore  supply.  On  June  1,  1940, 
Sachs,  Briggs,  and  Urey  met  with  Admiral  Bowen  to  discuss 
approaching  officials  of  the  Union  Miniere  of  the  Belgian  Congo. 
Such  an  approach  was  made  shortly  afterwards  by  Sachs. 

3.8.  The  general  status  of  the  problem  was  discussed  by  a 
special  advisory  group  called  together  by  Briggs  at  the  National 
Bureau  of  Standards  on  June  15,  1940.  This  meeting  was  attended 
by  Briggs,  Urey,  M.  A.  Tuve,  Wigner,  Breit,  Fermi,  Szilard, 
and  Pegram.  "After  full  discussion,  the  recommendation  of  the 
group  to  the  Uranium  Committee  was  that  funds  should  be 

Administrative  History  to  1941  49 

sought  to  support  research  on  the  uranium-carbon  experiment 
along  two  lines: 

(A)  further  measurements  of  the  nuclear  constants  involved 
in  the  proposed  type  of  reaction ; 

(B)  experiments  with  amounts  of  uranium  and  carbon  equal 
to  about  one  fifth  to  one  quarter  of  the  amount  that  could 
be  estimated  as  the  minimum  in  which  a  chain  reaction 
would  sustain  itself. 

"It  was  estimated  that  about  $40,000  would  be  necessary  for 
further  measurements  of  the  fundamental  constants  and  that 
approximately  $100,000  worth  of  metallic  uranium  and  pure 
graphite  would  be  needed  for  the  intermediate  experiment." 
(Quotations  from  memorandum  of  Pegram  to  Briggs,  dated 
August  14,  1940.) 


3.9.  Before  any  decisions  made  at  this  meeting  could  be  put 
into  effect,  the  organization  of  the  National  Defense  Research 
Committee  was  announced  in  June  1 940,  and  President  Roosevelt 
gave  instructions  that  the  Uranium  Committee  should  be  recon- 
stituted as  a  subcommittee  of  the  NDRC,  reporting  to  Vannevar 
Bush    (chairman,    NDRC).    The  membership    of    this    recon- 
stituted Uranium  Committee  was  as  follows:  Briggs,  Chairman; 
Pegram,  Urey,  Beams,  Tuve,  R.  Gunn  and  Breit.  On  authoriza- 
tion from  Briggs,  Breit  consulted  Wigner  and  Teller  frequently 
although  they  were  not  members  of  the  committee.  From  that 
time  until  the  summer  of  1941    this  committee  continued  in 
control  with  approximately  the  same  membership.  Its  recom- 
mendations were  transmitted  by  Briggs  to  the  NDRC,  and  suit- 
able  contracts   were   made   between   the   NDRC   and   various 
research  institutions.  The  funds,  however,  were  first  supplied  by 
the  Army  and  Navy,  not  from  regular  NDRC  appropriations. 


3.10.  The  first  contract  let  under   this  new  set-up  was  to 
Columbia  University  for  the  two  lines  of  work  recommended 

50  Administrative  History  to  1941 

at  the  June  15  meeting  as  described  above.  The  project  was 
approved  by  the  NDRC  and  the  first  NDRC  contract  (NDCrc-32) 
was  signed  November  8,  1940,  being  effective  from  November  1, 
1940,  to  November  1,  1941.  The  amount  of  this  contract  was 

3.11.  Only  very  small  expenditures  had  been  made  before 
the  contract  went  into  effect.  For  example,  about  $3,200  had 
been  spent  on  graphite  and  cadmium,  this  having  been  taken 
from  the  $6,000  allotted  by  the  Army  and  Navy  in  February, 

3.12.  We  shall  not  attempt  to  review  in  detail  the  other  con- 
tracts that  were  arranged  prior  to  December  1941.  Their  number 
and  total  amount  grew  gradually.  Urey  began  to  work  on  isotope 
separation  by  the  centrifuge  method  under  a  Navy  contract  in 
the  fall  of  1940.   Other  contracts  were  granted  to  Columbia 
University,   Princeton   University,   Standard   Oil  Development 
Company,   Cornell  University,   Carnegie   Institution  of  Wash- 
ington,   University   of  Minnesota,    Iowa   State   College,  Johns 
Hopkins  University,  National  Bureau  of  Standards,  University 
of  Virginia,  University  of  Chicago,  and  University  of  California 
in  the  course  of  the  winter  and  spring  of  1940-1941  until  by 
November   1941    the   total   number  of  projects   approved  was 
sixteen,  totalling  about  $300,000. 

3.13.  Scale  of  expenditure  is  at  least  a  rough  index  of  activity. 
It  is  therefore  interesting  to  compare  this  figure  with  those  in 
other  branches  of  war  research.  By  November  1941  the  total 
budget  approved  by  NDRC  for  the  Radiation  Laboratory  at 
the  Massachusetts  Institute  of  Technology  was  several  million 
dollars.  Even  a  relatively  small  project  like  that  of  Section  S  of 
Division  A  of  the  NDRC  had  spent  or  been  authorized  to  spend 
$136,000  on  work  that  proved  valuable  but  was  obviously  not 
potentially  of  comparable  importance  to  the  uranium  work. 


3.14.  The  Uranium  Committee  as  formed  in  the  summer  of 
1940  continued  substantially  unchanged  until  the  summer  of 

Administrative  History  to  1941  51 

1941.  At  that  time  the  main  committee  was  somewhat  enlarged 
and  subcommittees  formed  on  isotope  separation,  theoretical 
aspects,  power  production  and  heavy  water.*  It  was  thereafter 
called  the  Uranium  Section  or  the  S-l  Section  of  NDRC. 
Though  not  formally  disbanded  until  the  summer  of  1942,  this 
revised  committee  was  largely  superseded  in  December  1941 
(see  Chapter  V). 


3.15.  In  the  spring  of  1941,  Briggs,  feeling  that  an  impartial 
review  of  the  problem  was  desirable,  requested  Bush  to  appoint 
a  reviewing   committee.   Bush   then  formally  requested   F.   B. 
Jewett,    president   of  the    National   Academy   of  Sciences,    to 
appoint  such  a  committee.  Jewett  complied,  appointing  A.  H. 
Compton,  chairman;  W.  D.  Coolidge,  E.  O.  Lawrence,  J.  C. 
Slater,  J.  H.  Van  Vleck,  and  B.  Gherardi.  (Because  of  illness, 
Gherardi  was  unable  to  serve.)  This  committee  was  instructed  to 
evaluate  the  military  importance  of  the  uranium  problem  and 
to  recommend  the  level  of  expenditure  at  which  the  problem 
should  be  investigated. 

3.16.  This  committee  met  in  May  and  submitted  a  report. 
(This  report  and  the  subsequent  ones  will  be  summarized  in  the 
next  chapter.)  On  the  basis  of  this  report  and  the  oral  exposition 
by  Briggs  before  a  meeting  of  the  NDRC,  an  appropriation  of 
$267,000  was  approved  by  the  NDRC  at  its  meeting  of  July  18, 
1941,  and  the  probability  that  much  larger  expenditures  would 
be  necessary  was  indicated.  Bush  asked  for  a  second  report  with 
emphasis  on  engineering  aspects,   and  in  order  to  meet  this 
request  O.  E.  Buckley  of  the  Bell  Telephone  Laboratories  and 

*  Uranium  Section:  Briggs,  chairman;  Pegram,  vice-chairman;  S.  K. 
Allison,  Beams,  Breit,  E.  U.  Condon,  H.  D.  Smyth,  Urey. 

Separation  Subsection:  Urey,  chairman;  Beams. 

Power  Production  Subsection:  Pegram,  chairman;  Allison,  Fermi, 
Smyth,  Szilard. 

Heavy  Water  Subsection:  Urey,  chairman;  T.  H.  Chilton. 

Theoretical  Aspects  Subsection:  Fermi,  chairman;  Breit,  C.  H.  Eckart, 
Smyth,  Szilard,  J.  A.  Wheeler. 

52  Administrative  History  to  1941 

L.  W.  Chubb  of  the  Westinghouse  Electrical  and  Manufacturing 
Company  were  added  to  the  committee.  (Compton  was  in  South 
America  during  the  summer  and  therefore  did  not  participate  in 
the  summer  meetings  of  the  committee.)  The  second  report  was 
submitted  by  Coolidge.  As  a  result  of  new  measurements  of  the 
fission  cross  section  of  U-235  and  of  increasing  conviction  that 
isotope  separation  was  possible,  in  September  1941,  Compton 
and  Lawrence  suggested  to  J.  B.  Conant  of  NDRC,  who  was 
working  closely  with  Bush,  that  a  third  report  was  desirable.  Since 
Bush  and  Conant  had  learned  during  the  summer  of  1941  that 
the  British  also  felt  increasingly  optimistic,  the  committee  was 
asked  to  make  another  study  of  the  whole  subject.  For  this  pur- 
pose the  committee  was  enlarged  by  the  addition  of  W.  K.  Lewis, 
R.  S.  Mulliken,  and  G.  B.  Kistiakowsky.  This  third  report  was 
submitted  by  Compton  on  November  6,  1941. 


3.17.  Beginning  in  1940  there  was  some  interchange  of  infor- 
mation with  the  British  and  during  the  summer  of  1941  Bush 
learned  that  they  had  been  reviewing  the  whole  subject  in  the 
period  from  April  to  July.  They  too  had  been  interested  in 
the  possibility  of  using  plutonium;  in  fact,  a  suggestion  as  to  the 
advisability  of  investigating  plutonium  was  contained  in  a  letter 
from  J.  D.  Cockcroft  to  R.  H.  Fowler  dated  December  28,  1940. 
Fowler,  who  was  at  that  time  acting  as  British  scientific  liaison 
officer  in  Washington,  passed  Cockcroft's  letter  on  to  Lawrence. 
The  British  never  pursued  the  plutonium  possibility,  since  they 
felt    their    limited    manpower    should    concentrate    on    U-235. 
Chadwick,  at  least,  was  convinced  that  a  U-235  bomb  of  great 
destructive  power  could  be  made,  and  the  whole  British  group 
felt  that  the  separation  of  U-235  by  diffusion  was  probably  feasible. 

3.18.  Accounts  of  British  opinion,  including  the  first  draft  of 
the  British  report  reviewing  the  subject,  were  made  available  to 
Bush  and  Conant  informally  during  the  summer  of  1941,  although 
the  official  British  report  of  July  1 5  was  first  transmitted  to  Conant 
by  G.  P.  Thomson  on  October  3.  Since,  however,  the  British 

Administrative  History  to  1941  53 

review  was  not  made  available  to  the  committee  of  the  National 
Academy  of  Sciences,  the  reports  by  the  Academy  committee 
and  the  British  reports  constituted  independent  evaluations  of  the 
prospects  of  producing  atomic  bombs. 

3.19.  Besides  the  official  and  semi-official  conferences,  there 
were  many  less  formal  discussions  held,  one  of  these  being  stimu- 
lated by  M.  L.  E.  Oliphant  of  England  during  his  visit  to  this 
country  in  the  summer  of  1 941 .  As  an  example  of  such  informal 
discussion  we  might  mention   talks  among   Conant,  Compton, 
and  Lawrence  at  the  University  of  Chicago  semicentennial  cele- 
bration in  September  1941.  The  general  conclusion  was  that  the 
program  should  be  pushed;  and  this  conclusion  in  various  forms 
was  communicated  to  Bush  by  a  number  of  persons. 

3.20.  In  the  fall  of  1 941  Urey  and  Pegram  were  sent  to  England 
to  get  first-hand  information  on  what  was  being  done  there. 
This  was  the  first  time  that  any  Americans  had  been  to  England 
specifically  in  connection  with  the  uranium  problem.  The  report 
prepared  by  Urey  and  Pegram  confirmed  and  extended  the 
information  that  had  been  received  previously. 


3.21.  As  a  result  of  the  reports  prepared  by  the  National 
Academy  committee,  by  the  British,  and  by  Urey  and  Pegram, 
and  of  the  general  urging  by  a  number  of  physicists,  Bush,  as 
Director  of  the  Office  of  Scientific  Research  and  Development 
(of  which  NDRG  is  a  part),  decided  that  the  uranium  work 
should  be  pushed  more  aggressively. 

3.22.  Before  the  National  Academy  issued  its  third  report  and 
before  Pegram  and  Urey  visited  England,  Bush  had  taken  up 
the  whole  uranium  question  with  President  Roosevelt  and  Vice- 
President  Wallace.  He  summarized  for  them  the  British  views, 
which  were  on  the  whole  optimistic,  and  pointed  out  the  un- 
certainties of  the  predictions.  The  President  agreed  that  it  was 
desirable  to  broaden  the  program,  to  provide  a  different  organiza- 
tion, to  provide  funds  from  a  special  source,  and  to  effect  com- 
plete interchange  of  information  with  the  British.  It  was  agreed 

54  Administrative  History  to  1941 

to  confine  discussions  of  general  policy  to  the  following  group: 
The  President,  Vice-President,  Secretary  of  War,  Chief  of  Staff, 
Bush,  and  Conant.  This  group  was  often  referred  to  as  the  Top 
Policy  Group. 

3.23.  By  the  time  of  submission  of  the  National  Academy's 
third  report  and  the  return  of  Urey  and  Pegram  from  England, 
the  general  plan  of  the  reorganization  was  beginning  to  emerge. 
The  Academy's  report  was  more  conservative  than  the  British 
report,  as  Bush  pointed  out  in  his  letter  of  November  27,  1941,  to 
President  Roosevelt.  It  was,  however,  sufficiently  optimistic  to 
give  additional  support  to  the  plan  of  enlarging  the  work.  The 
proposed  reorganization  was  announced  at  a  meeting  of  the 
Uranium  Section  just  before  the  Pearl  Harbor  attack  and  will  be 
described  in  Chapter  V. 


3.24.  In  March  1939,  only  a  few  weeks  after  the  discovery  of 
uranium  fission,  the  possible  military  importance  of  fission  was 
called  to  the  attention  of  the  government.  In  the  autumn  of  1939 
the  first  government  committee  on  uranium  was  created.  In  the 
spring  of  1 940  a  mechanism  was  set  up  for  restricting  publication 
of  significant  articles  in  this  field.  When  the  NDRC  was  set  up 
in  June  1940,  the  Uranium  Committee  was  reconstituted  under 
the  NDRC.  However,  up  to  the  autumn  of  1941  total  expendi- 
tures were  relatively  small.  In  December  1941,  after  receipt  of 
the  National  Academy  report  and  information  from  the  British, 
the  decision  was  made  to  enlarge  and  reorganize  the  program. 



4.1.  In  Chapter  II  the  general  problems  involved  in  producing 
a  chain  reaction  for  military  purposes  were  described.  Early  in 
the  summer  of  1 940  the  questions  of  most  immediate  importance 

(1)  Could  any  circumstances  be  found  under  which  the  chain 
reaction  would  go? 

(2)  Could  the  isotope  U-235  be  separated  on  a  large  scale? 

(3)  Could   moderator   and   other   materials   be   obtained   in 
sufficient  purity  and  quantity? 

Although  there  were  many  subsidiary  problems,  as  will  appear 
in  the  account  of  the  progress  made  in  the  succeeding  eighteen 
months,  these  three  questions  determined  the  course  of  the  work. 



4.2.  In  June  1940,  nearly  all  work  on  the  chain  reaction  was 
concentrated    at    Columbia    under    the    general    leadership    of 
Pegram,  with  Fermi  and  Szilard  in  immediate  charge.  It  had 
been  concluded  that  the  most  easily  produced  chain  reaction 
was  probably  that  depending  on  thermal  neutron  fission  in  a 
heterogeneous  mixture  of  graphite  and  uranium.  In  the  spring 
of  1940  Fermi,  Szilard  and  H.  L.  Anderson  had  improved  the 
accuracy  of  measurements  of  the  capture  cross  section  of  carbon 
for  neutrons,  of  the  resonance  (intermediate-speed)  absorption 
of  neutrons  by  U-238,  and  of  the  slowing  down  of  neutrons  in 


56  Progress  to  December  1941 

4.3.  Pegram,  in  a  memorandum  to  Briggs  on  August  14,  1940, 
wrote,  "It  is  not  very  easy  to  measure  ,  these  quantities  with 
accuracy  without  the  use  of  large  quantities  of  material.  The 
net  results  of  these  experiments  in  the  spring  of  1940  were  that 
the  possibility  of  the  chain  reaction  was  not  definitely  proven, 
while  it  was  still  further  from  being  definitely  disproven.  On 
the  whole,  the  indications  were  more  favorable  than  any  con- 
clusions  that   could   fairly   have   been   claimed   from   previous 

4.4.  At  a  meeting  on  June  15  (see  Chapter  III)  these  results 
were  discussed  and  it  was  recommended  that  (A)  further  measure- 
ments be  made  on  nuclear  constants,  and  (B)  experiments  be 
made  on  lattices  of  uranium  and  carbon  containing  amounts  of 
uranium  from  one  fifth  to  one  quarter  the  estimated  critical 


4.5.  Pegram's  report  of  February  15,  1941   shows  that  most 
of  the  work  done  up  to  that  time  was  on  (A),  while  (B),  the 
so-called    intermediate    experiment,    was    delayed    by    lack    of 

4.6.  Paraphrasing  Pegram's  report,  the  main  progress  was  as 

(a)  The  slowing  down  of  neutrons  in  graphite  was  investigated  by 
studying  the  intensity  of  activation  of  various  detectors  (rhodium, 
indium,  iodine)  placed  at  various  positions  inside  a  rectangular 
graphite  column  of  dimensions  3X3X8  feet  when  a  source  of 
neutrons  was  placed  therein.  By  suitable  choice  of  cadmium 
screens  the  effects  of  resonance*  and  thermal  neutrons  were 
investigated  separately,  f  A  mathematical  analysis,  based  on  dif- 

*  See  footnote  p.  58. 

f  The  presence  of  neutrons  can  be  detected  by  ionization  chambers  or 
counters  or  by  the  artificial  radioactivity  induced  in  various  metal  foils. 
(See  Appendix  1.)  The  response  of  each  of  these  detectors  depends  on  the 
particular  characteristics  of  the  detector  and  on  the  speed  of  the  neutrons 
(e.g.,  neutrons  of  about  1.5  volts  energy  are  particularly  effective  in  acti- 
vating indium).  Furthermore,  certain  materials  have  very  large  absorption 

(Progress  to  December  1941  57 

fusion  theory,  of  the  experimental  data  made  it  possible  to  predict 
the  results  to  be  expected  in  various  other  arrangements.  These 
results,  coupled  with  theoretical  studies  of  the  diffusion  of  thermal 
neutrons,  laid  a  basis  for  future  calculations  of  the  number  of 
thermal  and  resonance  neutrons  to  be  found  at  any  point  in  a 
graphite  mass  of  given  shape  when  a  given  neutron  source  is 
placed  at  a  specified  position  within  or  near  the  graphite.  , 

(b)  The  number  of  neutrons  emitted  in  fission.  The  experiments  on 
slowing  down  neutrons  showed  that  high-energy  (high-speed) 
neutrons  such  as  those  from  fission  were  practically  all  reduced 
to  thermal  energies  (low  speeds)  after  passing  through  40  cm  or 
more  of  graphite.  A  piece  of  uranium  placed  in  a  region  where 
thermal  neutrons  are  present  absorbs  the  thermal  neutrons  and 
— as   fission   occurs — re-emits   fast   neutrons,    which  are   easily- 
distinguished  from  the  thermal  neutrons.  By  a  series  of  measure- 
ments  with  and   without   uranium   present   and   with  various 
detectors  and  absorbers,  it  is  possible  to  get  a  value  for  the  con- 
stant 77,  the  number  of  neutrons  emitted  per  thermal  neutron 
absorbed  by  uranium.  This  is  not  the  number  of  neutrons  emitted 
per  fission,  but  is  somewhat  smaller  than  that  number  since  not 
every  absorption  causes  fission. 

(c)  Lattice  theory.   Extensive  calculations  were  made  on  the 
probable  number  of  neutrons  escaping  from  lattices  of  various 
designs  and  sizes.  This  was  fundamental  for  the  so-called  inter- 
mediate experiment,  mentioned  above  as  item  (B). 


4.7.  Early  in  1941  interest  in  the  general  chain-reaction 
problem  by  individuals  at  Princeton,  Chicago,  and  California 
led  to  the  approval  of  certain  projects  at  those  institutions. 
Thereafter  the  work  of  these  groups  was  coordinated  with  the 
work  at  Columbia,  forming  parts  of  a  single  large  program. 

cross  sections  for  neutrons  of  particular  ranges  of  speed  (e.g.,  cadmium  for 
thermal  neutrons).  Thus  measurements  with  different  detectors  with  or 
without  various  absorbers  give  some  indication  of  both  the  number  of 
neutrons  present  and  their  energy  distribution.  However,  the  state  of  the  art 
of  such  measurements  is  rather  crude. 

58  Progress  to  December  1941 


4.8.  In  Chapter  II  it  is  stated  that  there  were  advantages  in  a 
lattice  structure  or  "pile"  with  uranium  concentrated  in  lumps 
regularly  distributed  in  a  matrix  of  moderator.  This  was  the 
system  on  which  the  Columbia  group  was  working.  As  is  so  often 
the  case,  the  fundamental  idea  is  a  simple  one.  If  the  uranium 
and  the  moderator  are  mixed  homogeneously,  the  neutrons  on 
the  average  will  lose  energy  in  small  steps  between  passages 
through  the  uranium  so  that  in  the  course  of  their  reduction  to 
thermal  velocity  the  chance  of  their  passing  through  uranium  at 
any  given  velocity,  e.g.,  at  a  velocity  corresponding  to  resonance 
absorption,  is  great.  But,  if  the  uranium  is  in  large  lumps  spaced 
at  large  intervals  in  the  moderator,  the  amounts  of  energy  lost  by 
neutrons  between  passages  from  one  lump  of  uranium  to  another 
will  be  large  and  the  chance  of  their  reaching  a  uranium  lump 
with  energy  just  equal  to  the  energy  of  resonance  absorption  is 
relatively  small.  Thus  the  chance  of  absorption  by  U-238  to 
produce  U-239,  compared  to  the  chance  of  absorption  as  thermal 
neutrons  to  cause  fission,  may  be  reduced  sufficiently  to  allow  a 
chain  reaction  to  take  place.  If  one  knew  the  exact  values  of  the 
cross  sections  of  each  uranium  isotope  for  each  type  of  absorption 
and  every  range  of  neutron  speed,  and  had  similar  knowledge  for 
the  moderator,  one  could  calculate  the  "optimum  lattice,"  i.e., 
the  best  size,  shape  and  spacing  for  the  lumps  of  uranium  in  the 
matrix  of  moderator.  Since  such  data  were  only  partially  known, 
a  direct  experimental  approach  appeared  to  be  in  order.  Conse- 
quently it  was  proposed   that  the  absorption  of  neutrons  by 
uranium  should  be  measured  under  conditions  similar  to  those  ex- 
pected in  a  chain-reacting  pile  employing  graphite  as  moderator. 

4.9.  Experiments  of  this  type  were  initiated  at  Columbia,  and 
were  continued  at  Princeton  in  February  1941.  Essentially  the 

*  The  term  "resonance  absorption"  is  used  to  describe  the  very  strong 
absorption  of  neutrons  by  U-238  when  the  neutron  energies  are  in  certain 
definite  portions  of  the  energy  region  from  0  to  1,000  electron  volts.  Such 
resonance  absorption  demonstrates  the  existence  of  nuclear  energy  levels  at 
corresponding  energies.  On  some  occasions  the  term  resonance  absorption 
is  used  to  refer  to  the  whole  energy  region  in  the  neighborhood  of  such  levels. 

Progress  to  December  1941  59 

experiment  consisted  of  studying  the  absorption  of  neutrons  in 
the  energy  range  extending  from  a  few  thousand  electron  volts 
down  to  a  fraction  of  an  electron  volt  (thermal  energies),  the 
absorption  taking  place  in  different  layers  of  uranium  or  uranium 
oxide  spheres  embedded  in  a  pile  of  graphite. 

4.10.  In  these  experiments,  a  source  of  neutrons  was  provided 
by  a  mean  of  protons  (accelerated  by  a  cyclotron)  impinging  on 
a  beryllium  target.  (The  resulting  yield  of  neutrons  was  equiva- 
lent to  the  yield  from  a  radium-beryllium  source  of  about  3,500 
curies  strength.)  The  neutrons  thus  produced  had  a  wide,  con- 
tinuous, velocity  distribution.  They  proceeded  from  this  source 
into  a  large  block  of  graphite.  By  placing  the  various  uranium  or 
uranium-oxide    spheres   inside    the   graphite   block   at   various 
positions    representing    increasing    distances    from    the    source, 
absorption  of  neutrons  of  decreasing  average  speeds  down  to 
thermal  speeds  was  studied.  It  was  found  that  the  total  absorption 
of  neutrons  by  such  spheres  could  be  expressed  in  terms  of  a 
"surface"  effect  and  a  "mass"  effect. 

4.11.  These  experiments,  involving  a  variety  of  sphere  sizes, 
densities,  and  positions  were  continued  until  the  spring  of  1942, 
when  most  of  the  group  was  moved  to  Chicago.  Similar  experi- 
ments performed  at  a  later  date  at  the  University  of  Indiana  by 
A.  C.  G.  Mitchell  and  his  co-workers  have  verified  and  in  some 
cases  corrected  the  Princeton  data,  but  the  Princeton  data  were 
sufficiently  accurate  by  the  summer  of  1941  to  be  used  in  plan- 
ning the  intermediate-pile  experiments  and  the  subsequent  ex- 
periments on  operating  piles. 

4.12.  The   experimental   work   on   resonance   absorption   at 
Princeton  was  done  by  R.  R.  Wilson,  E.  C.  Creutz,  and  their 
collaborators,  under  the  general  leadership  of  H.  D.  Smyth;  they 
benefited  from  the  constant  help  of  Wigner  and  Wheeler  and 
frequent  conferences  with  the  Columbia  group. 


4.13.  About  July  1941  the  first  lattice  structure  of  graphite  and 
uranium  was  set  up  at  Columbia.  It  was  a  graphite  cube  about 

60  Progress  to  December  1941 

8  feet  on  an  edge,  and  contained  about  7  tons  of  uranium  oxide 
in  iron  containers  distributed  at  equal  intervals  throughout  the 
graphite.  A  preliminary  set  of  measurements  was  made  on  this 
structure  in  August  1941.  Similar  structures  of  somewhat  larger 
size  were  set  up  and  investigated  during  September  and  October, 
and  the  so-called  exponential  method  (described  below)  of 
determining  the  multiplication  factor  was  developed  and  first 
applied.  This  work  was  done  by  Fermi  and  his  assistants,  H.  L. 
Anderson,  B.  Feld,  G.  Weil,  and  W.  H.  Zinn. 

4.14.  The  multiplication-factor  experiment  is  rather  similar 
to  that  already  outlined  for  the  determination  of  17,  the  number 
of  neutrons  produced  per  thermal  neutron  absorbed.  A  radium- 
beryllium  neutron  source  is  placed  near  the  bottom  of  the  lattice 
structure  and  the  number  of  neutrons  is  measured  at  various 
points  throughout  the  lattice.  These  numbers  are  then  compared 
with  the  corresponding  numbers  determined  when  no  uranium 
is  present  in  the  graphite  mass.   Evidently  the  absorption  of 
neutrons  by  U-238  to  produce  U-239  tends  to  reduce  the  number 
of  neutrons,   while  the  fissions  tend  to  increase  the  number. 
The  question  is:  Which  predominates?  or,  more  precisely,  Does 
the  fission  production  of  neutrons  predominate  over  all  neutron- 
removal  processes  other  than  escape?  Interpretation  of  the  experi- 
mental data  on  this  crucial  question  involves  many  corrections, 
calculations,  and  approximations,  but  all  reduce  in  the  end  to  a 
single  number,  the  multiplication  factor  k. 


4.15.  The  whole  success  or  failure  of  the  uranium  project 
depended  on  the  multiplication  factor  k,  sometimes  called  the 
reproduction  factor.  If  k  could  be  made  greater  than   1    in  a 
practical  system,  the  project  would  succeed;  if  not,  the  chain 
reaction  would  never  be  more  than  a  dream.  This  is  clear  from 
the  following  discussion,  which  applies  to  any  system  containing 
fissionable  material.  Suppose  that  there  is  a  certain  number  of 
free  neutrons  present  in  the  system  at  a  given  time.  Some  of 
these   neutrons  will   themselves   initiate   fissions   and  will   thus 

Progress  to  December  1941  61 

directly  produce  new  neutrons.  The  multiplication  factor  k  is  the 
ratio  of  the  number  of  these  new  neutrons  to  the  number  of  free 
neutrons  originally  present.  Thus,  if  in  a  given  pile  comprising 
uranium,  carbon,  impurities,  containers,  etc.,  100  neutrons  are 
produced  by  fission,  some  will  escape,  some  will  be  absorbed  in 
the  uranium  without  causing  fission,  some  will  be  absorbed  in 
the  carbon,  in  the  containers  or  in  impurities,  and  some  will 
cause  fission,  thereby  producing  more  neutrons.*  If  the  fissions 
are  sufficiently  numerous  and  sufficiently  effective  individually, 
more  than  100  new  neutrons  will  be  produced  and  the  system  is 
chain  reacting.  If  the  number  of  new  neutrons  is  105,  k  =  1.05. 
But  if  the  number  of  new  neutrons  per  100  initial  ones  is  99, 
k  =  .99  and  no  chain  reaction  can  maintain  itself. 

4.16.  Recognizing    that   the    intermediate   or    "exponential" 
experiment  described  above  was  too  small  to  be  chain  reacting, 
we  see  that  it  was  a  matter  of  great  interest  whether  any  larger 
pile  of  the  same  lattice  structure  would  be  chain  reacting.  This 
could  be  determined  by  calculating  what  the  value  of  k  would  be 
for  an  infinitely  large  lattice  of  this  same  type.  In  other  words, 
the  problem  was  to  calculate  what  the  value  of  k  would  be  if  no 
neutrons  leaked  away  through  the  sides  of  the  pile.  Actually  it  is 
found  that,  once  a  chain-reacting  system  is  well  above  the  critical 
size — say  two  or  three  times  as  great — and  is  surrounded  by  what 
is  called  a  reflector,  the  effective  value  of  k  differs  very  little  from 
that  for  infinite  size  provided  that  k  is  near  1.00.  Consequently, 
it   has   become   customary   to   characterize   the   chain-reaction 
potentialities  of  different  mixtures  of  metal  and  moderator  by 
the  value  of  k*  the  multiplication  constant  obtained  by  assuming 
infinite  size  of  pile. 

4.17.  The  value  of  k*  as  reported  by  Fermi  to  the  Uranium 
Section  in  the  fall  of  1941  was  about  0.87.  This  was  based  on 
results  from  the  second  Columbia  intermediate  experiment.  All 
agreed  that  the  multiplication  factor  could  be  increased  by  greater 
purity  of  materials,  different  lattice  arrangements,  etc.   None 
could  say  with  certainty  that  it  could  be  made  greater  than  one. 

*  See  drawing  facing  p.  35. 

62  Progress  to  December  1941 


4.18.  At  about  the  same  time  that  the  work  on  resonance 
absorption  was  started  at  Princeton,  S.  K.  Allison,  at  the  sugges- 
tion of  A.  H.  Compton,  began  work  at  Chicago  under  a  contract 
running  from  January  1,  1941  to  August  1,  1941.   The  stated 
objectives  of  the  work  were  to  investigate  (a)  the  increase  in 
neutron  production  when  the  pile  is  enclosed  in  a  beryllium 
envelope  or  "reflector,"  and  (b)  the  cross  sections  of  beryllium. 
A  new  contract  was  authorized  on  July  18,  1941,  to  run  to  June 
30,  1942.  This  stated  the  somewhat  broader  objective  of  investi- 
gating uranium-beryllium-carbon  systems  generally.  The  appro- 
priations involved  were  modest:  $9,500  for  the  first  contract,  and 
$30,000  for  the  second  contract. 

4.19.  As  has  already  been  pointed  out  in  Chapter  II,  beryllium 
has  desirable  qualities  as  a  moderator  because  of  its  low  atomic 
weight  and  low  neutron-absorption  cross  section;  there  was  also 
the  possibility  that  a  contribution  to  the  number  of  neutrons 
would  be  realized  from  the  (n,  2n)  reaction  in  beryllium.  The 
value  of  the  cross  section  was  not  precisely  known;  furthermore 
it  was  far  from  certain  that  any  large  amount  of  pure  beryllium 
could  be  obtained.  Allison's  problem  was  essentially  similar  to  the 
Columbia  problem,  except  for  the  use  of  beryllium  in  place  of 
graphite.  Because  of  the  scarcity  of  beryllium  it  was  suggested 
that  it  might  be  used  in  conjunction  with  graphite  or  some  other 
moderator,  possibly  as  a  reflector. 

4.20.  In  the  Chicago  experiments,  neutrons  produced  with  the 
aid  of  a  cyclotron  were  caused  to  enter  a  pile  of  graphite  and 
beryllium.   Allison  made   a  number  of  measurements   on   the 
slowing  down  and  absorption  by  graphite  which  were  valuable 
checks  on  similar  experiments  at  Columbia.  He  finally  was  able 
to  obtain  enough  beryllium  to  make  significant  measurements 
which  showed  that  beryllium  was  a  possible  moderator  com- 
parable to  graphite.  However,  beryllium  was  not  in  fact  used  at 
all  extensively  in  view  of  the  great  difficulty  of  producing  it  in 
quantity  in  the  required  structural  forms. 

Progress  to  December  1941  63 

4.21.  This  Chicago  project  as  described  above  became  part 
of  the  Metallurgical  Laboratory  project  established  at  the  Uni- 
versity of  Chicago  early  in  1942. 


4.22.  Both  the  intermediate  experiments  at  Columbia  and  the 
continued    resonance-absorption    work    at    Princeton    required 
skilful  theoretical  interpretation.  Fermi  worked  out  the  theory 
of  the  "exponential"  pile  and  Wigner  the  theory  of  resonance 
absorption;    both   these   men   were   constantly   conferring   and 
contributing  to  many  problems.  Wheeler  of  Princeton,  Breit  of 
Wisconsin,  and  Eckart  of  Chicago — to  mention  only  a  few — also 
made  contributions  to  general  pile  theory  and  related  topics. 
Altogether  one  can  say  that  by  the  end  of  1941  the  general  theory 
of  the  chain  reaction  for  slow  neutrons  was  almost  completely 
understood.  It  was  the  numerical  constants  and  technological 
possibilities  that  were  still  uncertain. 

4.23.  On  the  theory  of  a  fast-neutron  reaction  in  U-235  a  good 
deal  of  progress  had  also  been  made.  In  particular,  new  estimates 
of  the  critical  size  were  made,  and  it  was  predicted  that  possibly 
10  per  cent  of  the  total  energy  might  be  released  explosively. 
On  this  basis  one  kilogram  of  U-235  would  be  equivalent  to 
2,000  tons  of  TNT.  The  conclusions  are  reviewed  below  in  con- 
nection with  the  National  Academy  Report.  It  is  to  be  remem- 
bered that  there  are  two  factors  involved:  (1)  how  large  a  fraction 
of  the  available  fission  energy  will  be  released  before  the  reaction 
stops;  (2)  how  destructive  such  a  highly  concentrated  explosion 
will  be. 


4.24.  In  Chapter  I  mention  is  made  of  the  suggestion  that  the 
element  94,  later  christened  plutonium,  would  be  formed  by 
beta-ray  disintegrations  of  U-239  resulting  from  neutron  absorp- 
tion by  U-238  and  that  plutonium  would  probably  be  an  alpha- 
particle  emitter  of  long  half-life  and  would  undergo  fission  when 
bombarded  by  neutrons.   In  the  summer  of  1940  the  nuclear 

64  Progress  to  December  1941 

physics  group  at  the  University  of  California  in  Berkeley  was 
urged  to  use  neutrons  from  its  powerful  cyclotron  for  the  pro- 
duction of  plutonium,  and  to  separate  it  from  uranium  and 
investigate  its  fission  properties.  Various  pertinent  experiments 
were  performed  by  E.  Se'gre,  G.  T.  Seaborg,  J.  W.  Kennedy, 
and  A.  C.  Wahl  at  Berkeley  prior  to  1941  and  were  reported  by 
E.  O.  Lawrence  to  the  National  Academy  Committee  (see  below) 
in  May  1941  and  also  in  a  memorandum  that  was  incorporated 
in  the  Committee's  second  report  dated  July  11,  1941.  It  will  be 
seen  that  this  memorandum  includes  one  important  idea  not 
specifically  emphasized  by  others  (paragraph  1.58),  namely, 
the  production  of  large  quantities  of  plutonium  for  use  in  a  bomb. 

4.25.  We  quote  from  Lawrence's  memorandum  as  follows: 
"Since  the  first  report  of  the  National  Academy  of  Sciences  Com- 
mittee on  Atomic  Fission,  an  extremely  important  new  possibility 
has  been  opened  for  the  exploitation  of  the  chain  reaction  with 
unseparated  isotopes  of  uranium.  Experiments  in  the  Radiation 
Laboratory  of  the  University  of  California  have  indicated  (a)  that 
element  94  is  formed  as  a  result  of  capture  of  a  neutron  by 
uranium  238  followed  by  two  successive  beta-transformations, 
and  furthermore  (b)  that  this  transuranic  element  undergoes 
slow  neutron  fission  and  therefore  presumably  behaves  like 
uranium  235. 

"It  appears  accordingly  that,  if  a  chain  reaction  with  unsepa- 
rated isotopes  is  achieved,  it  may  be  allowed  to  proceed  violently 
for  a  period  of  time  for  the  express  purpose  of  manufacturing 
element  94  in  substantial  amounts.  This  material  could  be  ex- 
tracted by  ordinary  chemistry  and  would  presumably  be  the 
equivalent  of  uranium  235  for  chain  reaction  purposes. 

"If  this  is  so,  the  following  three  outstanding  important  possi- 
bilities are  opened: 

"1.  Uranium  238  would  be  available  for  energy  production, 
thus  increasing  about  one  hundred  fold  the  total  atomic  energy 
obtainable  from  a  given  quantity  of  uranium. 

"2.  Using  element  94  one  may  envisage  preparation  of  small 
chain  reaction  units  for  power  purposes  weighing  perhaps  a 

Progress  to  December  1941  65 

hundred  pounds  instead  of  a  hundred  tons  as  probably  would  be 
necessary  for  units  using  natural  uranium. 

"3.  If  large  amounts  of  element  94  were  available  it  is  likely 
that  a  chain  reaction  with  fast  neutrons  could  be  produced.  In 
such  a  reaction  the  energy  would  be  released  at  an  explosive 
rate  which  might  be  described  as  'super  bomb.' " 


4.26.  As  previously  stated,  the  fragments  resulting  from  fission 
are  in  most  cases  unstable  nuclei,  that  is,  artificially  radioactive 
materials.   It  is  common  knowledge  that  the  radiations  from 
radioactive  materials  have  deadly  effects  akin  to  the  effects  of 

4.27.  In  a  chain-reacting  pile  these  radioactive  fission  products 
build  up  as  the  reaction  proceeds.  (They  have,  in  practice,  turned 
out  to  be  the  most  troublesome  feature  of  a  reacting  pile.)  Since 
they  differ  chemically  from  the  uranium,  it  should  be  possible 
to  extract  them  and  use  them  like  a  particularly  vicious  form  of 
poison  gas.  This  idea  was  mentioned  in  the  National  Academy 
report   (see  paragraph  4.48)   and  was  developed  in  a  report 
written  December  10,  1941,  by  E.  Wigner  and  H.  D.  Smyth, 
who  concluded  that  the  fission  products  produced  in  one  day's 
run  of  a  100,000  kw  chain-reacting  pile  might  be  sufficient  to 
make  a  large  area  uninhabitable. 

4.28.  Wigner  and  Smyth  did  not  recommend  the  use  of  radio- 
active  poisons  nor  has  such  use  been  seriously  proposed  since 
by  the  responsible   authorities,   but  serious  consideration  was 
given  to  the  possibility  that  the  Germans  might  make  surprise 
use  of  radioactive  poisons,  and  accordingly  defensive  measures 
were  planned. 



4.29.  In  Chapter  I  the  attribution  of  thermal-neutron  fission 
of  uranium  to  the  U-235  isotope  was  mentioned  as  being  experi- 

66  Progress  to  December  1941 

mentally  established.  This  was  done  by  partly  separating  minute 
quantities  of  the  uranium  isotopes  in  A.  O.  Nier's  mass  specto- 
graph  and  then  studying  the  nuclear  properties  of  the  samples. 
Additional  small  samples  were  furnished  by  Nier  in  the  summer 
of  1941  and  studied  by  N.  P.  Heydenburg  and  others  at  M.  A. 
Tuve's  laboratory  at  the  Department  of  Terrestrial  Magnetism 
of  the  Carnegie  Institution  of  Washington.  But  results  of  such 
experiments  were  still  preliminary,  and  it  was  evident  that 
further  study  of  larger  and  more  completely  separated  samples 
was  desirable. 

4.30.  The  need  of  larger  samples  of  U-235  stimulated  E.  O. 
Lawrence  at  Berkeley  to  work  on  electromagnetic  separation. 
He  was  remarkably  successful  and  by  December  6,  1941  reported 
that  he  could  deposit  in  one  hour  one  microgram  of  U-235  from 
which  a  large  proportion  of  the  U-238  had  been  removed. 

4.31.  Previously,  at  a  meeting  of  the  Uranium  Committee, 
Smyth  of  Princeton  had  raised  the  question  of  possible  large-scale 
separation  of  isotopes  by  electromagnetic  means  but  had  been 
told  that  it  had  been  investigated  and  was  considered  impossible. 
Nevertheless,   Smyth  and   Lawrence   at  a  chance   meeting   in 
October  1941  discussed  the  problem  and  agreed  that  it  might  yet 
be  possible.  Smyth  again  raised  the  question  at  a  meeting  of  the 
Uranium  Committee  on  December  6  and  at  the  next  meeting 
(December  18,  1941)  there  was  a  general  discussion  of  large-scale 
electromagnetic  methods  in  connection  with  Lawrence's  report 
of  his  results  already  mentioned.  The  consequences  of  this  dis- 
cussion are  reported  in  Chapter  XI. 


4.32.  Though  we  have  made  it  clear  that  the  separation  of 
U-235  from  U-238  might  be  fundamental  to  the  whole  success  of 
the  project,  little  has  been  said  about  work  in  this  field.  Such  work 
had  been  going  on  since  the  summer  of  1940  under  the  general 
direction  of  H.  C.  Urey  at  Columbia.  Since  this  part  of  the  ura- 
nium work  was  not  very  much  affected  by  the  reorganization  in 

Progress  to  December  1941  67 

December  1941,  a  detailed  account  of  the  work  is  reserved  for 
Chapters  IX  and  X.  Only  a  summary  is  presented  here. 

4.33.  After    careful   review   and    a   considerable    amount   of 
experimenting  on  other  methods,  it  had  been  concluded  that  the 
two  most  promising  methods  of  separating  large  quantities  of 
U-235  from  U-238  were  by  the  use  of  centrifuges  and  by  the  use 
of  diffusion  through  porous  barriers.  In  the  centrifuge,  the  forces 
acting  on  the  two  isotopes  are  slightly  different  because  of  their 
differences  in  mass.  In  the  diffusion  through  barriers,  the  rates  of 
diffusion  are  slightly  different  for  the  two  isotopes,  again  because 
of  their  differences  in  mass.  Each  method  required  the  uranium  to 
be  in  gaseous  form,  which  was  an  immediate  and  serious  limita- 
tion since  the  only  suitable  gaseous  compound  of  uranium  then 
known  was  uranium  hexafluoride.  In  each  method  the  amount 
of  enrichment  to  be  expected  in  a  single  production  unit  or  "stage" 
was  very  small;  this  indicated  that  many  successive  stages  would 
be  necessary  if  a  high  degree  of  enrichment  was  to  be  attained. 

4.34.  By  the  end  of  1941  each  method  had  been  experimentally 
demonstrated  in  principle;  that  is,  single-stage  separators  had 
effected  the  enrichment  of  the  U-235  on  a  laboratory  scale  to 
about  the  degree  predicted  theoretically.  K.  Cohen  of  Columbia 
and  others  had  developed  the  theory  for  the  single  units  and  for 
the  series  or  "cascade"  of  units  that  would  be  needed.  Thus  it 
was  possible  to  estimate  that  about  5,000  stages  would  be  neces- 
sary for  one  type  of  diffusion  system  and  that  a  total  area  of 
many  acres  of  diffusion  barrier  would  be  required  in  a  plant 
separating  a  kilogram  of  U-235  each  day.  Corresponding  cost 
estimates  were  tens  of  millions  of  dollars.  For  the  centrifuge  the 
number  of  stages  would  be  smaller,  but  it  was  predicted  that  a 
similar  production  by  centrifuges  would  require  22,000  separately 
driven,  extremely  high-speed  centrifuges,  each  three  feet  in  length 
at  a  comparable  cost. 

4.35.  Of  course,  the  cost  estimates  could  not  be  made  accu- 
rately since  the  technological  problems  were  almost  completely 
unsolved,  but  these  estimates  as  to  size  and  cost  of  plant  did  serve 
to  emphasize  the  magnitude  of  the  undertaking. 

68  Progress  to  December  1941 


4.36.  In  September  1940,  P.  H.  Abelson  submitted  to  Briggs  a 
17-page  memorandum  suggesting  the  possibility  of  separating 
the  isotopes  of  uranium  by  thermal  diffusion  in  liquid  uranium 
hexafluoride.  R.  Gunn  of  the  Naval  Research  Laboratory  was 
also  much  interested  in  the  uranium  problem  and  was  appointed 
a  member  of  the  Uranium  Committee  when  it  was  reorganized 
under  the  NDRC  in  the  summer  of  1940.  As  a  result  of  Abelson's 
suggestion  and  Gunn's  interest,  work  was  started  on  thermal  dif- 
fusion at  the  National  Bureau  of  Standards.   This  work  was 
financed  by  funds  from  the  Navy  Department  and  in  1940  was 
transferred  to  the  Naval  Research  Laboratory,  still  under  the 
direction  of  Abelson,  where  it  was  continued. 

4.37.  We  shall  discuss  the  thermal-diffusion  work  further  in  a 
later  chapter,  but  we  may  mention  here  that  significant  results 
had  already  been  obtained  by  the  end  of  1941  and  that  in  Janu- 
ary 1942,  using  a  single  separation  column,  a  separation  factor 
had  been  obtained  which  was  comparable  or  superior  to  the  one 
obtained  up  to  that  time  in  preliminary  tests  on  the  diffusion 
and  centrifuge  methods. 


4.38.  It  was  pointed  out  in  Chapter  II  that  deuterium  ap- 
peared very  promising  as  a  moderator  because  of  its  low  absorp- 
tion and  good  slowing-down  property  but  unpromising  because 
of  its  scarcity.  Interest  in  a  deuterium  moderator  was  stimulated 
by  experimental  results  obtained  in  Berkeley  demonstrating  that 
the  deuterium  absorption  cross  section  for  neutrons  was,  in  fact, 
almost  zero.  Since  oxygen  has  a  very  low  absorption  coefficient 
for  neutrons,  it  was  usually  assumed  that  the  deuterium  would  be 
used  combined  with  oxygen,   that  is,  in  the  very  convenient 
material:  heavy  water.  Work  at  Columbia  on  possible  methods 
of  large-scale   concentration  of  heavy  water  was   initiated   in 
February  1941  under  the  direction  of  H.  C.  Urey  (under  an 
OSRD  contract).  Early  in  1941,  R.  H.  Fowler  of  England  re- 

Progress  to  December  1941  69 

ported  the  interest  of  the  British  group  in  a  moderator  of  deu- 
terium in  the  form  of  heavy  water  and  their  conviction  that  a 
chain  reaction  would  go  in  relatively  small  units  of  uranium  and 
heavy  water. 

4.39.  Urey  and  A.  von  Grosse  had  already  been  considering 
the  concentration  of  heavy  water  by  means  of  a  catalytic  ex- 
change reaction  between  hydrogen  gas  and  liquid  water.  This 
process  depends  on  the  fact  that,  when  isotopic  equilibrium  is 
established  between  hydrogen  gas  and  water,  the  water  contains 
from  three  to  four  times  as  great  a  concentration  of  deuterium  as 
does  the  hydrogen  gas.   During  1941,   this  exchange  reaction 
between  water  and  hydrogen  was  investigated  at  Columbia  and 
in  the  Frick  Chemical  Laboratory  at  Princeton  and  extensive 
work  was  done  toward  developing  large-scale  methods  of  pro- 
ducing materials  suitable  for  catalyzing  the  reaction. 

4.40.  The  further  development  of  this  work  and   of  other 
methods  of  producing  heavy  water  are  discussed  in  Chapter  IX. 
Like  the  other  isotope-separation  work  at  Columbia,  this  work 
was  relatively  unaffected  by  the  reorganization  in  December 
1941.  It  is  mentioned  in  preliminary  fashion  here  to  indicate 
that  all  the  principal  lines  of  approach  were  under  investigation 
in  1941. 


4.41.  By  the  end  of  1941  not  very  much  progress  had  been 
made  in  the  production  of  materials  for  use  in  a  chair-reacting 
system.  The  National  Bureau  of  Standards  and  the  Columbia 
group  were  in  contact  with  the  Metal  Hydrides  Company  of 
Beverly,    Massachusetts.    This   company   was   producing   some 
uranium  in  powdered  form,  but  efforts  to  increase  its  production 
and  to  melt  the  powdered  metal  into  solid  ingots  had  not  been 
very  successful. 

4.42.  Similarly,  no  satisfactory  arrangement  had  been  made 
for  obtaining  large  amounts  of  highly  purified  graphite.  The 
graphite  in  use  at  Columbia  had  been  obtained  from  the  U.  S. 
Graphite  Company  of  Saginaw,  Michigan.  It  was  of  high  purity 

70  Progress  to  December  1941 

for  a  commercial  product,  but  it  did  contain  about  one  part  in 
500,000  of  boron,  which  was  undesirable. 

4.43.  Largely  through  the  interest  of  Allison  the  possibility  of 
increasing  the  production  of  beryllium  had  been  investigated  to 
the  extent  of  ascertaining  that  it  would  be  difficult  and  expensive, 
but  probably  possible. 

4.44.  Though  little  progress  had  been  made  on  procurement, 
much  progress  had  been  made  on  analysis.  The  development  of 
sufficiently  accurate  methods  of  chemical  analysis  of  the  materials 
used  has  been  a  problem  of  the  first  magnitude  throughout  the 
history  of  the  project,  although  sometimes  overshadowed  by  the 
more   spectacular   problems   encountered.    During   this   period 
C.  J.  Rodden  and  others  at  the  National  Bureau  of  Standards 
were  principally  responsible  for  analyses;  H.  T.  Beans  of  Columbia 
also  cooperated.  By  1942  several  other  groups  had  started  analyt- 
ical sections  which  have  been  continuously  active  ever  since. 

4.45.  To  summarize,  by  the  end  of  1941  there  was  no  evidence 
that  procurement  of  materials  in  sufficient  quantity  and  purity 
was  impossible,  but  the  problems  were  far  from  solved. 


4.46.  Prior  to  the  autumn  of  1941  there  had  been  some  ex- 
change of  reports  with  the  British  and  some  discussion  with 
British  scientific  representatives  who  were  here  on  other  business. 
In  September  1941,  it  was  decided  that  Pegram  and  Urey  should 
get  first-hand  information  by  a  trip  to  England.  They  completed 
their  trip  in  the  first  week  of  December  1941. 

4.47.  In  general,  work  in  England  had  been  following  much 
the  same  lines  as  in  this  country.  As  to  the  chain-reaction  problem, 
their  attention  had  focussed  on  heavy  water  as  a  moderator 
rather  than  graphite;  as  to  isotope  separation,  they  had  done  ex- 
tensive work  on  the  diffusion  process  including  the  general  theory 
of  cascades.  Actually  the  principal  importance  of  this  visit  and 
other  interchanges  during  the  summer  of  1941  lay  not  in  accurate 

Progress  to  December  1941  71 

scientific  data  but  in  the  general  scientific  impressions.  The 
British,  particularly  J.  Chadwick,  were  convinced  that  a  U-235 
chain  reaction  could  be  achieved.  They  knew  that  several  kilo- 
grams of  heavy  water  a  day  were  being  produced  in  Norway, 
and  that  Germany  had  ordered  considerable  quantities  of  par- 
affin to  be  made  using  heavy  hydrogen;  it  was  difficult  to  imagine 
a  use  for  these  materials  other  than  in  work  on  the  uranium 
problem.  They  feared  that  if  the  Germans  got  atomic  bombs 
before  the  Allies  did,  the  war  might  be  over  in  a  few  weeks.  The 
sense  of  urgency  which  Pegram  and  Urey  brought  back  with  them 
was  of  great  importance. 


4.48.  The  appointment  of  a  National  Academy  committee  was 
mentioned  in  Chapter  III.  The  committee's  first  report  in  May 
1941  mentioned  (a)  radioactive  poisons,  (b)  atomic  power,  and 
(c)  atomic  bombs,  but  the  emphasis  was  on  power.  The  second 
report  stressed  the  importance  of  the  new  results  on  plutonium, 
but  was  not  specific  about  the  military  uses  to  which  the  fission 
process  might  be  put.  Both  these  reports  urged  that  the  project 
be  pushed  more  vigorously. 

4.49.  The  third  report  (November  6,  1941)  was  specifically 
concerned  with  the  "possibilities  of  an  explosive  fission  reaction 
with  U-235."  Although  neither  of  the  first  two  National  Academy 
reports  indicated  that  uranium  would  be  likely  to  be  of  decisive 
importance  in  the  present  war,  this  possibility  was  emphasized 
in  the  third  report.  We  can  do  no  better  than  quote  portions  of 
this  report. 

"Since  our  last  report,  the  progress  toward  separation  of  the 
isotopes  of  uranium  has  been  such  as  to  make  urgent  a  considera- 
tion of  (1)  the  probability  of  success  in  the  attempt  to  produce  a 
fission  bomb,  (2)  the  destructive  effect  to  be  expected  from  such 
a  bomb,  (3)  the  anticipated  time  before  its  development  can  be 
completed  and  production  be  underway,  and  (4)  a  preliminary 
estimate  of  the  costs  involved." 

72  Progress  to  December  1941 

"1.  Conditions  for  a  fission  bomb.  A  fission  bomb  of  superlatively 
destructive  power  will  result  from  bringing  quickly  together  a  sufficient 
mass  of  element  U-235.  This  seems  to  be  as  sure  as  any  untried 
prediction  based  upon  theory  and  experiment  can  be.  Our 
calculations  indicate  further  that  the  required  masses  can  be 
brought  together  quickly  enough  for  the  reaction  to  become 
efficient  .  .  . 

"2.  Destructive  effect  of  fission  bombs,  (a)  Mass  of  the  bomb.  The 
mass  of  U-235  required  to  produce  explosive  fission  under  appropriate  con- 
ditions can  hardly  be  less  than  2  kg  nor  greater  than  100  kg.  These  wide 
limits  reflect  chiefly  the  experimental  uncertainty  in  the  capture 
cross  section  of  U-235  for  fast  neutrons  .  .  .  (b)  Energy  released 
by  explosive  fission.  Calculations  for  the  case  of  masses  properly- 
located  at  the  initial  instant  indicate  that  between  1  and  5  per 
cent  of  the  fission  energy  of  the  uranium  should  be  released  at  a 
fission  explosion.  This  means  from  2  to  10  X  108  kilocalories  per 
kg  of  uranium  235.  The  available  explosive  energy  per  kg  of  uranium 
is  thus  equivalent  to  about  300  tons  of  TNT. 

"3.  Time  required  for  development  and  production  of  the  necessary 
U-235.  (a)  Amount  of  uranium  needed.  Since  the  destructiveness  of 
present  bombs  is  already  an  important  factor  in  warfare,  it  is 
evident  that,  if  the  destructiveness  of  the  bombs  is  thus  increased 
10,000-fold,  they  should  become  of  decisive  importance. 

"The  amount  of  uranium  required  will,  nevertheless,  be  large. 
If  the  estimate  is  correct  that  500,000  tons  of  TNT  bombs  would 
be  required  to  devastate  Germany's  military  and  industrial 
objectives,  from  1  to  10  tons  of  U-235  will  be  required  to  do  the  same  job. 

"(b)  Separation  of  U-235.  The  separation  of  the  isotopes  of  uranium 
can  be  done  in  the  necessary  amounts.  Several  methods  are  under 
development,  at  least  two  of  which  seem  definitely  adequate, 
and  are  approaching  the  stage  of  practical  test.  These  are  the 
methods  of  the  centrifuge  and  of  diffusion  through  porous  bar- 
riers. Other  methods  are  being  investigated  or  need  study  which 
may  ultimately  prove  superior,  but  are  now  farther  from  the 
engineering  stage. 

"(c)   Time  required  for  production  of  fission  bombs.  An  estimate  of 

Progress  to  December  1941  73 

time  required  for  development,  engineering  and  production  of 
fission  bombs  can  be  made  only  very  roughly  at  this  time. 

"If  all  possible  effort  is  spent  on  the  program,  one  might 
however  expect  fission  bombs  to  be  available  in  significant 
quantity  within  three  or  four  years. 

"4.  Rough  estimate  of  costs.  (The  figures  given  in  the  Academy 
report  under  this  heading  were  recognized  as  only  rough  esti- 
mates since  the  scientific  and  engineering  data  to  make  them 
more  precise  were  not  available.  They  showed  only  that  the 
undertaking  would  be  enormously  expensive  but  still  in  line  with 
other  war  expenditures.)" 

4.50.  The  report  then  goes  on  to  consider  immediate  require- 
ments and  desirable  reorganization. 


4.51.  At  the  end  of  Chapter  I  we  summarized  the  knowledge 
of  nuclear  fission  as  of  June  1940,  and  in  Chapter  II  we  stated 
the  outstanding  problems  as  of  the  same  date.  In  the  light  of 
these  statements  we  wish  to  review  the  eighteen  months'  progress 
that  has  just  been  recounted.  The  tangible  progress  was  not  great. 
No  chain  reaction  had  been  achieved;  no  appreciable  amount 
of  U-235  had  been  separated  from  U-238;  only  minute  amounts  of 
Pu-239  had  been  produced;  the  production  of  large  quantities  of 
uranium  metal,  heavy  water,  beryllium,  and  pure  graphite  was 
still  largely  in  the  discussion  stage.  But  there  had  been  progress. 
Constants  were  better  known;  calculations  had  been  checked 
and  extended;  guesses  as  to  the  existence  and  nuclear  properties 
of  Pu-239  had  been  verified.  Some  study  had  been  made  of  engi- 
neering problems,  process  effectiveness,  costs,  and  time  schedules. 
Most  important  of  all,  the  critical  size  of  the  bomb  had  been 
shown  to  be  almost  certainly  within  practical  limits.  Altogether 
the  likelihood  that  the  problems  might  be  solved  seemed  greater 
in  every  case  than  it  had  in  1 940.  Perhaps  more  important  than 
the  actual  change  was  the  psychological  change.  Possibly  Wigner, 
Szilard,  and  Fermi  were  no  more  thoroughly  convinced  that 
atomic  bombs  were  possible  than  the>r  had  been  in  1940,  but 

74  Progress  to  December  1941 

many  other  people  had  become  familiar  with  the  idea  and  its 
possible  consequences.  Apparently,  the  British  and  the  Germans, 
both  grimly  at  war,  thought  the  problem  worth  undertaking. 
Furthermore,  the  whole  national  psychology  had  changed. 
Although  the  attack  at  Pearl  Harbor  was  yet  to  come,  the  im- 
pending threat  of  war  was  much  more  keenly  felt  than  before, 
and  expenditures  of  effort  and  money  that  would  have  seemed 
enormous  in  1940  were  considered  obviously  necessary  precau- 
tions in  December  1941.  Thus  it  was  not  surprising  that  Bush  and 
his  associates  felt  it  was  time  to  push  the  uranium  project  vigor- 
ously. For  this  purpose,  there  was  created  an  entirely  new 
administrative  organization  which  will  be  described  in  the  next 

HISTORY  1942-1945 

5.1.  In  Chapter  III  the  administrative  history  of  the  uranium 
work  up  to  December  1 941  was  reviewed.  Chapter  IV  reported 
the  progress  of  the  scientific  work  up  to  the  same  date.  The 
present  chapter  describes  the  administrative  reorganization  that 
took  place  in  December  1941  and  various  changes  that  occurred 
after  that  time. 


5.2.  Two  major  decisions  were  required  in  the  further  plan- 
ning of  the  uranium  or  atomic-bomb  program.  These  decisions 
were  made  by  Vannevar  Bush,  Director  of  the  Office  of  Scientific 
Research   and   Development    (which   included   NDRC),    after 
conference  with  various  scientists  and  administrators  concerned. 
(See  Chapter  III.)  The  decisions  were:  first,  that  the  possibility 
of  obtaining  atomic  bombs  for  use  in  the  present  war  was  great 
enough  to  justify  an  "all  out"  effort  for  their  development;  second, 
that  the  existing  organization,   the  NDRC   Uranium  Section 
(known  as  the  S-l  Section,  and  consisting  of  L.  J.  Briggs,  chair- 
man; G.  B.  Pegram,  vice-chairman;  H.  T.  Wensel,  technical 
aide;  S.  K.  Allison,  J.  W.  Beams,  G.  Breit,  E.  U.  Condon,  R. 
Gunn,  H.  D.  Smyth,  and  H.  C.  Urey)  was  not  properly  organized 
for  such  an  effort. 

5.3.  At  a  meeting  of  the  National  Defense  Research  Committee 
on  November  28,  1941,  Dr.  Bush  explained  why  he  felt  that  it 
was  desirable  to  set  up  the  uranium  program  outside  NDRC. 
The  members  of  NDRC  agreed  to  a  transfer.  Accordingly,  the 
NDRC  as  an  organization  had  no  further  connection  with  the 


76  Administrative  History  1942-1945 

uranium  program,  which  was  administered  for  some  time 
thereafter  by  the  OSRD  directly  through  an  OSRD  S-l  Section, 
and  later  through  an  OSRD  S-l  Executive  Committee. 

5.4.  At  a  meeting  of  the  S-l  Section  of  OSRD  on  December  6, 
1941,  J.  B.  Conant,  speaking  for  Bush,  announced  the  proposed 
"all  out"  effort  and  the  reorganization  of  the  group.  The  S-l 
Section  itself  had  not  been  formally  consulted  on  the  proposed 
reorganization,  but  there  is  no  doubt  that  most  of  its  members 
were  strongly  in  favor  of  the  new  proposals.  The  membership  of 
the  reorganized  S-l  Section  was  as  follows:  J.  B.  Conant,  repre- 
sentative of  V.  Bush;  L.  J.  Briggs,  chairman;  G.  B.  Pegram,  vice- 
chairman;  A.  H.  Compton,  program  chief;  E.  O.  Lawrence, 
program  chief;  H.  C.  Urey,  program  chief;  E.  V.  Murphree, 
chairman  of  the  separately  organized  Planning  Board;  H.  T. 
Wensel,  technical  aid;  S.  K.  Allison,  J.  W.  Beams,  G.  Breit, 
E.  U.  Condon,  H.  D.  Smyth. 


5.5.  At  the  time  the  S-l  Section  was  reorganized,  Bush  also 
set  up  a  Planning  Board  to  be  responsible  for  the  technical  and 
engineering  aspects  of  the  work,  for  procurement  of  materials 
and  for  construction   of  pilot  plants  and  full-size  production 
plants.  This  Planning  Board  consisted  of  E.  V.  Murphree  (chair- 
man), W.  K.  Lewis,  L.  W.  Chubb,  G.  O.  Curme,  Jr.,  and  P.  C. 


5.6.  It  was  arranged  that  contracts  for  the  scientific  parts  of 
the  work  would  be  recommended  to  Bush  not  by  the  full  S-l 
Section  but  by  Briggs  and  Conant  after  conferences  with  the 
program  chiefs  involved  and  that  recommendations  on  engineer- 
ing contracts  would  be  made  to  Bush  by  the  Planning  Board. 
(The  contracts  which  had  been   made  on   behalf  of  the   old 
Uranium  Section  had  been  administered  through  the  NDRC.) 

Administrative  History  1942-1945  77 

Contracts  for  the  development  of  diffusion  and  centrifuge  sepa- 
ration processes  were  to  be  recommended  by  the  Planning  Board, 
which  would  be  responsible  for  the  heavy-water  production 
program  also.  Bush  stated  that  the  Planning  Board  "will  be 
responsible  for  seeing  to  it  that  we  have  plans  on  which  to  proceed 
with  the  next  step  as  expeditiously  as  possible." 

5.7.  The  scientific  aspects  of  the  work  were  separated  from  the 
procurement  and  engineering  phases.   The   Program   Chiefs — 
H.  C.  Urey,  E.  O.  Lawrence,  and  A.  H.  Compton — were  to  have 
charge  of  the  scientific  aspects.  Initially  it  was  proposed  that  Urey 
should  have  charge  of  the  separation  of  isotopes  by  the  diffusion 
and  the  centrifuge  methods  and  of  the  research  work  on  the 
production  of  heavy  water.  Lawrence  was  to  have  charge  of  the 
initial  production  of  small  samples  of  fissionable  elements,  of 
quantity  production  by  electromagnetic-separation  methods,  and 
of  certain  experimental  work  relating  to  the  properties  of  the 
plutonium  nucleus.  Compton  was  to  have  charge  of  fundamental 
physical  studies  of  the  chain  reaction  and  the  measurement  of 
nuclear  properties  with  especial  reference  to  the  explosive  chain 
reaction.  As  an  afterthought,  he  was  authorized  to  explore  also 
the   possibility   that   plutonium   might   be   produced   in   useful 
amounts  by  the  controlled  chain-reaction  method.  It  was  under- 
stood, however,  that  this  division  of  responsibility  was  to  be  more 
precisely  defined  in  later  conferences.   (The  written  records  of 
that  period  do  not  always  give  adequate  accounts  of  what  was  in 
the  minds  of  the  men  concerned.  In  deference  to  security  re- 
quirements, references  to  the  importance  of  plutonium  and  even 
to  the  bomb  itself  were  often  omitted  entirely.) 

5.8.  The  effect  of  the  reorganization  was  to  put  the  direction 
of  the  projects  in  the  hands  of  a  small  group  consisting  of  Bush, 
Conant,   Briggs,    Compton,    Urey,    Lawrence,    and   Murphree. 
Theoretically,  Compton,  Lawrence,  Urey,  and  Murphree  were 
responsible  only  for  their  respective  divisions  of  the  program. 
Each    met    with    Conant    and    Briggs    or    occasionally    with 
Bush   to    discuss   his    specific    problems,    or    even    the    overall 

78  Administrative  History  1942-1945 


5.9.  A  meeting  of  the  Top  Policy  Group,  consisting  of  Vice- 
President  Henry  A.  Wallace,  Secretary  of  War  Henry  L.  Stimson, 
and  Dr.  V.  Bush,  was  held  on  December  16, 1941 .  General  George 
C.  Marshall  and  Dr.  J.  B.  Conant,  also  members  of  the  group, 
were  absent;  Mr.  H.  L.  Smith  of  the  Budget  Bureau  attended. 
Bush  described  the  reorganization  that  was  in  progress  and  his 
plans  were  approved.  In  a  memorandum  to  Conant  describing 
this  meeting,  Bush  wrote,  "It  was  definitely  felt  by  the  entire 
group  that  OSRD  should  press  as  fast  as  possible  on  the  funda- 
mental physics  and  on  the  engineering  planning,  and  particularly 
on  the  construction  of  pilot  plants."  Bush  estimated  the  cost  of 
this  aspect  of  the  work  would  be  four  or  five  million  dollars,  and 
stated  the  Army  should  take  over  when  full-scale  construction 
was  started,  presumably  when  pilot  plants  were  ready.  He  sug- 
gested the  assignment  of  a  technically  trained  Army  officer  to 
become  familiar  with  the  general  nature  of  the  uranium  problem. 
It  was  made  clear  at  this  meeting  that  the  international  relations 
involved  were  in  the  hands  of  the  President,  with  Bush  responsible 
for  liaison  on  technical  matters  only. 


5.10.  On  December  18,  1941,  a  meeting  of  the  reorganized  S-l 
Section  was  held.  Conant  was  present  and  discussed  the  new 
policy,  which  called  for  an  all-out  effort.  He  emphasized  that 
such  an  effort  was  justified  only  by  the  military  value  of  atomic 
bombs  and  that  all  attention  must  be  concentrated  in  the  direc- 
tion of  bomb  development.  The  whole  meeting  was  pervaded 
by  an  atmosphere  of  enthusiasm  and  urgency.  Several  methods 
of  electromagnetic  separation  were  proposed  and  discussed,  and 
a  number  of  new  contracts  were  recommended. 


5.11.  Another  meeting  of  the  OSRD  S-l  Section  was  held  on 
January  16,  1942.  Informal  discussions  of  the  various  production 

Administrative  History  1942-1945  79 

methods  took  place,  and  tentative  estimates  were  made  as  to 
when  each  method  would  produce  results.  These  forecasts 
actually  were  no  more  than  guesses  since  at  that  time  the  scientific 
information  available  was  very  incomplete  and  the  problems  of 
applying  such  data  as  did  exist  to  the  construction  and  operation 
of  production  plants  had  hardly  been  approached. 


5.12.  In  the  middle  of  January  1942,  Compton  decided  to 
concentrate  the  work  for  which  he  was  responsible  at  the  Uni- 
versity of  Chicago.  The  Columbia  group  under  Fermi  and  its 
accumulated  material  and  equipment  and  the  Princeton  group 
which  had  been  studying  resonance  absorption  were  moved  to 
Chicago  in  the  course  of  the  spring.  Certain  smaller  groups  else- 
where   remained    active    under    Compton's    direction.    Under 
Lawrence    the    investigation    of    large-scale    electromagnetic 
separation  was  accelerated  at  the  University  of  California  at 
Berkeley  and  a  related  separation  project  was  started  at  Princeton. 
Research  and  development  on  the  diffusion  process  and  on  the 
production  of  heavy  water  continued  at  Columbia  under  Urey; 
under  the  general  supervision  of  Murphree,  the  centrifuge  work 
continued  at  the  University  of  Virginia  under  Beams  while  the 
Columbia  centrifuge  work  was  transferred  to  the  laboratories  of 
the  Standard  Oil  Development  Co.  at  Bayway,  New  Jersey. 

ON  MARCH  9,  1942 

5.13.  In  a  report  dated  February  20,  1942,  Conant  recom- 
mended that  all  phases  of  the  work  be  pushed  at  least  until  July 
1,  1942.  Similarly,  on  March  9,  1942,  Dr.  Bush  sent  a  report  to 
the  President  reflecting  general  optimism  but  placing  proper 
emphasis  on   the   tentative  nature   of  conclusions.   His  report 
contemplated  completion  of  the  project  in  1 944.  In  addition,  the 
report  contained  the  suggestion  that  the  Army  be  brought  in 
during  the  summer  of  1 942  for  construction  of  full-scale  plants. 

80  Administrative  History  1942-1945 


5.14.  The  entire  heavy-water  program  was  under  review  in 
March  and  April  1942.  The  reviews  followed  a  visit  to  the  United 
States  in  February  and  March  1942  by  F.  Simon,  H.  Halban, 
and  W.  A.  Akers  from  England.  In  a  memorandum  of  April  1, 
1942  addressed  to  Bush,  Conant  reviewed  the  situation  and  re- 
ported on  conferences  with  Compton  and  Briggs.  His  report 
pointed   out   that  extremely  large   quantities   of  heavy  water 
would  be  required  for  a  plutonium  production  plant  employing 
heavy  water  instead  of  graphite  as  a  moderator.  For  this  reason, 
he  reported  adversely  on  the  suggestion  that  Halban  be  invited 
to  bring  to  this  country  the  165  liters  of  heavy  water  which  he  then 
had  in  England. 

5.15.  In  a  memorandum  written  to  Bush  on  May  14,  1942 
(shortly  before  a  proposed  meeting  of  Program  Chiefs),  Conant 
estimated  that  there  were  five  separation  or  production  methods 
which  were  about  equally  likely  to  succeed:  the  centrifuge,  dif- 
fusion, and  electromagnetic  methods  of  separating  U-235;  the 
uranium-graphite  pile  and  the  uranium-heavy-water  pile  methods 
of  producing  plutonium.  All  were  considered  about  ready  for 
pilot  plant  construction  and  perhaps  even  for  preliminary  design 
of  production  plants.  If  the  methods  were  to  be  pushed  to  the 
production  stage,  a  commitment  of  five  hundred  million  dollars 
would  be  entailed.  Although  it  was  too  early  to  estimate  the 
relative  merits  of  the  different  methods  accurately,  it  was  pre- 
sumed that  some  methods  would  prove  to  be  more  rapid  and 
efficient  than  others.  It  was  feared,  however,  that  elimination 
of  any  one  method  might  result  in  a  serious  delay.  It  was  thought 
that  the  Germans  might  be  some  distance  ahead  of  the  United 
States  in  a  similar  program. 

5.16.  Conant  emphasized  a  question  that  has  been  crucial 
throughout  the  development  of  the  uranium  project.  The  ques- 
tion was  whether  atomic  bombs  would  be  decisive  weapons  or 
merely  supplementary  weapons.  If  they  were  decisive,  there  was 
virtually  no  limit  to  the  amount  of  effort  and  money  that  should 

Administrative  History  1942-1945  81 

be  put  into  the  work.  But  no  one  knew  how  effective  the  atomic 
bombs  would  be. 


5.17.  In  May  1942,  Conant  suggested  to  Bush  that  instead  of 
encouraging  members  of  the  section  individually  to  discuss  their 
own  phases  of  the  work  with  Conant  and  Briggs,  the  OSRD  S-l 
Section  should  meet  for  general  discussions  of  the  entire  program. 
Bush  responded  by  terminating  the  OSRD  S-l    Section  and 
replacing  it  with  the  OSRD  S-l  Executive  Committee,  consisting 
of  the  following:  J.  B.  Conant,  chairman,  L.  J.  Briggs,  A.  H. 
Compton,  E.  O.  Lawrence,  E.  V.  Murphree,  H.  C.  Urey.  H.  T. 
Wensel  and  I.  Stewart  were  selected  to  sit  with  the  Committee 
as  technical  aide  and  secretary  respectively. 

5.18.  The  following  members  of  the  old  OSRD  S-l  Section 
were  appointed  as  consultants  to  the  new  Committee:  S.  K. 
Allison,  J.  W.  Beams,  G.  Breit,  E.  U.  Condon,  H.  D.  Smyth. 

5.19.  The  functions  of  the  new  OSRD  S-l  Executive  Com- 
mittee were:  (a)  To  report  on  the  program  and  budget  for  the 
next  eighteen  months,  for  each  method,  (b)  To  prepare  recom- 
mendations as  to  how  many  programs  should  be  continued. 
(c)  To  prepare  recommendations  as  to  what  parts  of  the  program 
should  be  eliminated. 

5.20.  Recommendations   relative    to   matters   of  OSRD   S-l 
policy  and  relative  to  the  letting  of  OSRD  S-l  contracts  were 
made  on  the  basis  of  a  majority  vote  of  the  Committee.  Conant 
refrained  from  voting  except  in  case  of  a  tie  vote.  While  Bush 
alone  had  the  authority  to  establish  OSRD  policies  and  commit 
OSRD  funds,  he  ordinarily  followed  the  recommendations  of 
the  S-l  Executive  Committee. 

ON  JUNE  17,  1942 

5.21.  On    June   13,    1942,   Bush  and   Conant  sent  to  Vice- 
President  Henry  A.  Wallace,  Secretary  of  War  Henry  L.  Stimson, 

82  Administrative  History  1942-1945 

and  Chief  of  Staff  General  George  C.  Marshall  a  report  recom- 
mending detailed  plans  for  the  expansion  and  continuation  of 
the  atomic-bomb  program.  All  three  approved  the  report.  On 
June  17,  1942,  the  report  was  sent  by  Bush  to  the  President,  who 
also  approved.  The  report,  wrhich  is  too  long  to  present  in  full, 
contained  four  principal  parts,  which  dealt  with:  (a)  The  status 
of  the  development  as  appraised  by  the  senior  scientists;  (b) 
Recommendations  by  the  program  chiefs  and  Planning  Board; 
(c)  Comments  by  Bush,  Conant,  and  Maj.  Gen.  W.  D.  Styer;  (d) 
Recommendations  by  Bush  and  Conant.  We  may  paraphrase 
parts  (a)  and  (c)  as  follows: 

(a)  The  status  of  the  program.  (1)  It  was  clear  that  an  amount  of 
U-235  or  plutonium  comprising  a  number  of  kilograms  would  be 
explosive,  that  such  an  explosion  would  be  equivalent  to  several 
thousand  tons  of  TNT,  and  that  such  an  explosion  could  be 
caused  to  occur  at  the  desired  instant.  (2)  It  was  clear  that  there 
were  four  methods  of  preparing  the  fissionable  material  and  that 
all  of  these  methods  appeared  feasible;  but  it  was  not  possible  to 
state  definitely  that  any  given  one  of  these  is  superior  to  the  others. 
(3)  It  was  clear  that  production  plants  of  considerable  size  could 
be  designed  and  built.  (4)  It  seemed  likely  that,  granted  adequate 
funds  and  priorities,  full-scale  plant  operation  could  be  started 
soon  enough  to  be  of  military  significance. 

(c)  Comments  by  Bush,  Conant ,  and  General  Styer.  Certain  recom- 
mendations had  been  made  by  Lawrence,  Urey,  Compton,  and 
Murphree.  These  recommendations  had  been  reviewed  by  Bush, 
Conant,  and  General  Styer  (who  was  instructed  by  General 
Marshall  to  follow  the  progress  of  the  program)  and  their  com- 
ments concerning  the  program  were  as  follows:  (1)  If  four  sepa- 
rate methods  all  appeared  to  a  highly  competent  scientific  group 
to  be  capable  of  successful  application,  it  appeared  certain  that 
the  desired  end  result  could  be  attained  by  the  enemy,  provided 
he  had  sufficient  time.  (2)  The  program  as  proposed  obviously 
could  not  be  carried  out  rapidly  without  interfering  with  other 
important  matters,  as  regards  both  scientific  personnel  and 
critical  materials.  A  choice  had  to  be  made  between  the  military 

Administrative  History  1942-1945  83 

result  which  appeared  attainable  and  the  certain  interference 
with  other  war  activities.  (3)  It  was  unsafe  at  that  time,  in  view 
of  the  pioneering  nature  of  the  entire  effort,  to  concentrate  on 
only  one  means  of  obtaining  the  result.  (4)  It  therefore  appeared 
best  to  proceed  at  once  with  those  phases  of  the  program  which 
interfered  least  with  other  important  war  activities.  Work  on 
other  phases  of  the  program  could  proceed  after  questions  of 
interference  were  resolved. 

5.22.  The  June  13,  1942,  report  to  the  President  and  Bush's 
transmittal  letter  dated  June  17,  1942,  were  returned  to  Bush 
with  the  initialled  approval  of  the  President.  A  copy  of  the  report 
was  then  sent  by  Bush  to  General  Styer  on  June  19,  1942. 


5.23.  On  June  18,  1942,  Colonel  J.  C.  Marshall,  Corps  of 
Engineers,  was  instructed  by  the  Chief  of  Engineers  to  form  a 
new  district  in  the  Corps  of  Engineers  to  carry  on  special  work 
(atomic  bombs)  assigned  to  it.  This  district  was  designated  the 
Manhattan  District  and  was  officially  established  on  August  13, 
1942.  The  work  with  which  it  was  concerned  was  labeled,  for 
security  reasons,  the  "DSM  Project"  (Development  of  Substitute 


5.24.  On  September  17,  1942,  the  Secretary  of  War  placed 
Brigadier  General  L.  R.  Groves  of  the  Corps  of  Engineers  in 
complete  charge  of  all  Army  activities  relating  to  the  DSM 


5.25.  A  conference  was  held  on  September  23,  1942,  among 
those   persons   designated   by   the   President   to   determine   the 
general  policies  of  the  project,  and  certain  others.  Those  present 
were  Secretary  of  War  Henry  L.  Stimson,  Chief  of  Staff  General 

84  Administrative  History  1942-1945 

George  C.  Marshall,  Dr.  J.  B.  Conant,  Dr.  V.  Bush,  Major 
General  Brehon  Somervell,  Major  General  W.  D.  Styer,  and 
Brigadier  General  L.  R.  Groves.  (Vice-President  Henry  A. 
Wallace  was  unable  to  attend.)  A  Military  Policy  Committee 
was  appointed  consisting  of  Dr.  V.  Bush  as  Chairman  with  Dr. 
J.  B.  Conant  as  his  alternate,  Major  General  W.  D.  Styer,  and 
Rear  Admiral  W.  R.  Purnell.  General  Groves  was  named  to  sit 
with  the  committee  and  act  as  Executive  Officer  to  carry  out  the 
policies  that  were  determined.  The  duties  of  this  committee  were 
to  plan  military  policies  relating  to  materials,  research  and 
development,  production,  strategy,  and  tactics,  and  to  submit 
progress  reports  to  the  policy  group  designated  by  the  President. 

5.26.  The  appointment  of  the  Military  Policy  Committee  was 
approved  by  the  Joint  New  Weapons  Committee,  established  by 
the  U.  S.  Joint  Chiefs  of  Staff  and  consisting  of  Dr.  V.  Bush,  Rear 
Admiral  W.  R.  Purnell,  and  Brigadier  General  R.  G.  Moses. 

5.27.  The  creation  of  the  Military  Policy  Committee  in  effect 
placed  all  phases  of  the  DSM  Project  under  the  control  of  Dr. 
Bush,  Dr.  Conant,  General  Styer,  Admiral  Purnell,  and  General 

5.28.  The   OSRD   S-l    Executive   Committee   held   meetings 
about  once  every  month  from  June  1 942  to  May  1 943  and  once 
after  that  time,  in  September  1943.  These  meetings  were  normally 
attended  by  General  Groves,  after  September  1942,  and  Colonel 
Marshall,   and  frequently  by  representatives   of  the  industrial 
companies  concerned  with  the  production  plants.  Recommenda- 
tions  of  the   Committee  were  not   binding   but  were   usually 
followed.  Thus  it  served  as  an  advisory  body  to  Dr.  Bush  and 
General  Groves,  and  as  an  initial  liaison  group  between  the 
scientific,  industrial,  and  military  parts  of  the  DMS  Project. 
The  S-l  Executive  Committee  has  never  been  formally  dissolved, 
but  it  has  been  inactive  since  the  fall  of  1943. 

5.29.  The    procurement    and    engineering    functions    of   the 
Planning  Board  were  taken  over  by  the  Manhattan  District  in 
the  summer  of  1942  and  that  board  then  became  inactive. 

5.30.  By  the  spring  of  1943  it  was  felt  that  the  Manhattan 

Administrative  History  1942-1945  85 

District  was  in  a  position  to  take  over  research  and  development 
contracts  from  the  OSRD.  Such  a  transfer  was  effected  as  of 
May  1,  1943,  and  marked  the  end  of  the  formal  connection  of 
OSRD  with  the  uranium  project. 

5.31.  In  July  1943  Conant  and  R.  G.  Tolman  were  formally 
asked  by  General  Groves  to  serve  as  his  scientific  advisers.  They 
had  already  been  doing  so  informally  and  have  continued  to  do 
so.  Coordination  of  the  various  scientific  and  technical  programs 
was  accomplished  by  meetings  between  General  Groves  and  the 
leaders  of  the  various  projects,  in  particular,  Compton,  Lawrence, 
Oppenheimer  (see  Ghapter  XII),  and  Urey. 


5.32.  Since  1943  there  have  been  no  important  changes  in 
the  form  of  the  organization  and  few  of  importance  in  the  operat- 
ing personnel.  General  Groves  has  continued  to  carry  the  major 
responsibility  for  correlating  the  whole  effort  and  keeping  it 
directed  toward  its  military  objectives.  It  has  been  his  duty  to 
keep  the  various  parts  of  the  project  in  step,  to  see  that  raw 
materials  were  available  for  the  various  plants,  to  determine 
production  schedules,   to  make  sure  that  the  development  of 
bomb  design  kept  up  with  production  schedules,  to  arrange  for 
use  of  the  bombs  when  the  time  came,  and  to  maintain  an  ade- 
quate system  of  security.  In  discharging  these  duties  General 
Groves  has  had  the  help  of  his  tremendous  organization  made  up 
of  civilian  scientists  and  engineers  and  Engineer  officers  and 
enlisted  men.  Many  of  the  civilians  have  been  mentioned  already 
or  will  be  mentioned  in  later  chapters  dealing  with  particular 
projects.  Brigadier  General  T.  F.  Farrell  has  acted  as  General 
Groves'  deputy  in  the  important  later  phases  of  the  project. 
Colonel  K.  D.  Nichols,  the  District  Engineer  of  the  Manhattan 
District  with  his  headquarters  at  the  Clinton  Engineer  Works,  has 
been  connected  with  the  project  since  1942.  He  has  been  con- 
cerned with  the  research  and  production  problems  of  both  U-235 

86  Administrative  History  1942-1945 

and  plutonium  and  has  always  shown  exceptional  understanding 
of  the  technical  problems  and  their  relative  importance.  Two 
other  officers  who  should  be  mentioned  are  Colonel  F.  T.  Mat- 
thias and  Colonel  S.  L.  Warren.  Colonel  Matthias  has  discharged 
major  responsibilities  at  the  Hanford  Engineer  Works  in  an 
extremely  able  manner;  his  duties  have  been  concerned  with 
both  the  construction  and  operational  phases  of  the  project. 
Colonel  Warren  is  chief  of  the  Medical  Section  of  the  Manhattan 
District  and  therefore  has  had  ultimate  responsibility  for  health 
problems  in  all  parts  of  the  project. 


5.33.  By  the  end  of  1941  an  extensive  review  of  the  whole 
uranium  situation  had  been  completed.  As  a  result  of  this  review 
Bush  and  his  advisers  decided  to  increase  the  effort  on  the 
uranium  project  and  to  change  the  organization.  This  decision 
was  approved  by  President  Roosevelt.  From  January  1 942  until 
early  summer  of  1942  the  uranium  work  was  directed  by  Bush 
and  Conant  working  with  the  Program  Chiefs  and  a  Planning 
Board.  In  the  summer  of  1942  the  Army,  through  the  Corps  of 
Engineers,  was  assigned  an  active  part  in  the  procurement  and 
engineering  phases,  organizing  the  Manhattan  District  for  the 
purpose.  In  September  1942,  Dr.  Bush,  Dr.  Conant,  General 
Styer,  and  Admiral  Purnell  were  appointed  as  a  Military  Policy 
Committee  to  determine  the  general  policies  of  the  whole  project. 
Also  in  September,  General  Groves  was  appointed  to  take  charge 
of  all  Army  activities  of  the  project.  The  period  of  joint  OSRD 
and  Army  control  continued  through  April  1943  with  the  Army 
playing  an  increasingly  important  role  as  the  industrial  effort  got 
fully  under  way.  In  May  1943  the  research  contracts  were  trans- 
ferred to  the  Corps  of  Engineers;  the  period  of  joint  OSRD- Army 
control  ended  and  the  period  of  complete  Army  control  began. 

5.34.  Since  the  earliest  days  of  the  project,  President  Roosevelt 
had  followed  it  with  interest  and,  until  his  death,  continued 
to  study  and  approve  the  broad  programs  of  the  Military  Policy 

Administrative  History  1942-1945  87 

Committee.  President  Truman,  who  as  a  United  States  Senator 
had  been  aware  of  the  project  and  its  magnitude,  was  given  the 
complete  up-to-date  picture  by  the  Secretary  of  War  and  General 
Groves  at  a  White  House  conference  immediately  after  his 
inauguration.  Thereafter  the  President  gave  the  program  his 
complete  support,  keeping  in  constant  touch  with  the  progress. 

AT  CHICAGO  IN  1942 


6.1.  As  has  been  made  clear  in  Chapters  IV  and  V,  the  infor- 
mation accumulated  by  the  end  of  1941  as  to  the  possibility  of 
producing  an  atomic  bomb  was  such  as  to  warrant  expansion  of 
the  wrork,  and  this  expansion  called  for  an  administrative  reor- 
ganization. It  was  generally  accepted  that  there  was  a  very  high 
probability  that  an  atomic  bomb  of  enormous  destructive  power 
could  be  made,  either  from  concentrated  U-235  or  from  the  new 
element  plutonium.  It  was  proposed,  therefore,  to  institute  an 
intensive  experimental  and  theoretical  program  including  work 
both  on  isotope  separation  and  on  the  chain-reaction  problems. 
It  was  hoped  that  this  program  would  establish  definitely  whether 
or  not  U-235  could  be  separated  in  significant  quantities  from 
U-238,  either  by  electromagnetic  or  statistical  methods;  whether 
or  not  a  chain  reaction  could  be  established  with  natural  uranium 
or  its  compounds  and  could  be  made  to  yield  relatively  large 
quantities  of  plutonium;  and  whether  or  not  the  plutonium  so 
produced  could  be  separated  from  the  parent  material,  uranium. 
It  was  hoped  also  that  the  program  would  provide  the  theo- 
retical and  experimental  data  required  for  the  design  of  a  fast- 
neutron  chain-reacting  bomb. 

6.2.  As  has  been  explained  in  Chapter  V,  the  problems  of 
isotope  separation  had  been  assigned  to  groups  under  Lawrence 
and  Urey  while  the  remaining  problems  were  assigned  to  Comp- 
ton's  group,  which  was  organized  under  the  cryptically  named 
"Metallurgical  Laboratory"  of  the  University  of  Chicago.   In 
this  chapter  and  the  following  two  chapters  we  shall  describe  the 
work  of  the  Metallurgical  Laboratory  and  the  associated  labora- 
tories up  to  June  1945.  In  later  chapters  we  shall  discuss  isotope- 


Metallurgical  Project  in  1942  89 

separation  work  and  the  work  of  the  bomb  development  group, 
which  was  separated  from  the  Metallurgical  Laboratory  early 
in  1943. 

6.3.  It  would  be  futile  to  attempt  an  assessment  of  the  relative 
importance  of  the  contributions  of  the  various  laboratories  to  the 
overall  success  of  the  atomic-bomb  project.  This  report  makes  no 
such  attempt,  and  there  is  little  correlation  between  the  space 
devoted  to  the  work  of  a  given  group  and  the  ability  or  importance 
of  that  group.  In  deciding  which  subdivision  of  the  atomic-bomb 
project  should  be  discussed  first  and  most  fully,  we  have  been 
governed  by  criteria  of  general  interest  and  of  military  security. 
Some  developments  of  great  technical  importance  are  of  little 
ueneral  interest;  others  both  interesting  and  important  must  still 
be  kept  secret.  Such  criteria,  applied  to  the  objectives  and  ac- 
complishments of  the  various  laboratories  set  up  since  large-scale 
work  began,  favor  the  Metallurgical  Laboratory  as  the  part  of 
the  project  to  be  treated  most  completely. 


6.4.  In  accordance  with  the  general  objectives  just  outlined, 
the  initial  objectives  of  the  Metallurgical  Laboratory  were:  first, 
to  find  a  system  using  normal  uranium  in  which  a  chain  reaction 
would  occur;  second,  to  show  that,  if  such  a  chain  reaction  did 
occur,  it  would  be  possible  to  separate  plutonium  chemically  from 
the  other  material;  and,  finally,  to  obtain  the  theoretical  and 
experimental  data  for  effecting  an  explosive  chain  reaction  with 
either  U-235  or  with  plutonium.  The  ultimate  objective  of  the 
laboratory  was  to  prepare  plans  for  the  large-scale  production  of 
plutonium  and  for  its  use  in  bombs. 


6.5.  The  laboratory  had  not  only  to  concern  itself  with  its 
immediate  objectives  but  simultaneously  to  bear  in  mind  the 
ultimate  objectives  and  to  work  toward  them  on  the  assumption 
that  the  immediate  objectives  would  be  attained.  It  could  not 
wait  for  a  chain  reaction  to  be  achieved  before  studying  the 

90  Metallurgical  Project  in  1942 

chemistry  of  plutonium.  It  had  to  assume  that  plutonium  would 
be  separated  and  to  go  ahead  with  the  formulation  of  plans  for 
its  production  and  use.  Consequently  problems  were  continually 
redefined  as  new  information  became  available,  and  research 
programs  were  reassessed  almost  from  week  to  week.  In  a  general 
way  the  experimental  nuclear  physics  group  under  E.  Fermi  was 
primarily  concerned  with  getting  a  chain  reaction  going,  the 
chemistry  division  organized  by  F.  H.  Spedding  (later  in  turn 
under  S.  K.  Allison,  J.  Franck,  W.  C.  Johnson,  and  T.  Hogness) 
with  the  chemistry  of  plutonium  and  with  separation  methods, 
and  the  theoretical  group  under  E.  Wigner  with  designing  pro- 
duction piles.  However,  the  problems  were  intertwined  and  the 
various  scientific  and  technical  aspects  of  the  fission  process  were 
studied  in  whatever  group  seemed  best  equipped  for  the  particu- 
lar task.  In  March  1942,  Thomas  Moore  was  brought  in  to 
head  the  engineering  group.  Other  senior  men  in  this  group  were 
M.  C.  Leverett,  J.  A.  Wheeler  and  C.  M.  Cooper,  who  later 
succeeded  Moore  as  head  of  the  Technical  Division.  In  the 
summer  of  1942  the  importance  of  health  problems  became 
apparent  and  a  Health  Division  was  organized  under  Dr.  R.  S. 
Stone.  The  difficult  task  of  organizing  and  administering  a  re- 
search laboratory  growing  in  size  and  complexity  with  almost 
explosive  violence  was  carried  out  by  R.  L.  Doan  as  Laboratory 

6.6.  We  have  chosen  to  confine  this  chapter  to  the  work  of 
1942  because  a  self-sustaining  chain  reaction  was  first  achieved 
on  December  2  of  that  year,  at  a  time  when  the  whole  Chicago 
project  was  being  appraised  by  a  reviewing  committee  with  the 
members  particularly  selected  for  their  engineering  background.  * 
That  was  a  dramatic  coincidence  and  also  a  convenient  one  for 
purposes  of  this  report  since  either  incident  might  be  considered 
to  mark  the  end  of  an  epoch  at  the  Metallurgical  Laboratory. 
Furthermore,  in  preparation  for  the  reviewing  committee's  visit 

*  This  committee  was  composed  of  W.  K.  Lewis,  C.  H.  Greenewalt, 
T.  C.  Gary,  and  Roger  Williams.  E.  V.  Murphree  was  also  a  member  but 
due  to  illness  was  unable  to  participate. 

Metallurgical  Project  in  1942  91 

a  comprehensive  report  had  been  prepared.  That  report  was 
generally  known  as  the  "Feasibility  Report"  and  has  been  used 
extensively  in  preparing  this  chapter. 


6.7.  In  this  chapter  we  shall  present  the  material  in  the  order 
of  the  objectives  given  above.  In  Part  I  we  shall  discuss  progress 
towards   the   initial   objectives,    including    (a)    procurement   of 
materials,  (b)  the  experimental  proof  of  the  chain  reaction,  (c)  the 
chemistry  of  plutonium  and  some  of  the  problems  of  separation, 
(d)  some  of  the  types  of  auxiliary  experiments  that  were  per- 
formed, and  finally   (e)  the  "fast  neutron"  work.  Necessarily 
the  work  described  in  detail  is  only  a  sampling  of  the  large  amount 
of  theoretical  and  experimental  work  actually  performed.   In 
Part  II  we  shall  discuss  the  possibilities  that  were  considered  for 
production  piles  and  separation  methods,  and  the  specific  pro- 
posals made  in  November  1942. 




6.8.  It  has  been  made  clear  in  earlier  chapters  of  this  report 
that  the  procurement  of  materials  of  sufficient  purity  was  a  major 
part  of  the  problem.  As  far  as  uranium  was  concerned,  it  seemed 
likely  that  it  would  be  needed  in  highly  purified  metallic  form 
or  at  least  as  highly  purified  uranium  oxide.  The  other  materials 
which  were  going  to  be  needed  were  either  graphite,  heavy  water, 
or  possibly  beryllium.  It  was  clear  at  this  time  that,  however 
advantageous  heavy  water  might  be  as  a  moderator,  no  large 
quantities  of  it  would  be  available  for  months  or  years.  Beryllium 
seemed  less  advantageous  and  almost  as  difficult  to  get.  There- 
fore the  procurement  efforts  for  a  moderator  were  centered  on 
graphite.  As  has  been  explained  in  Chapter  V,  procurement  of 

92  Metallurgical  Project  in  1942 

uranium  and  graphite  was  not  primarily  the  responsibility  of 
the  Metallurgical  Laboratory  but  was  handled  through  E.  V. 
Murphree  and  others  on  the  "planning  board."  In  fact,  the 
obvious  interest  of  the  Metallurgical  Laboratory  in  the  problem 
led  to  continual  intervention  by  its  representatives.  A  great  deal 
of  the  credit  for  the  eventual  success  in  obtaining  materials  is  due 
to  N.  Hilberry  and  later  R.  L.  Doan,  always  supported  by 
A.  H.  Compton. 


6.9.  Obviously  there  would  be  no  point  in  undertaking  this 
whole  project  if  it  were  not  going  to  be  possible  to  find  enough 
uranium  for  producing  the  bombs.  Early  indications  were  favor- 
able, and  a  careful  survey  made  in  November  1942  showed  that 
immediate   delivery  could   be   made   of  adequate   tonnages   of 
uranium  ores. 


6.10.  At  the  end  of  1941  the  only  uranium  metal  in  existence 
was  a  few  grams  of  good  material  made  on  an  experimental  basis 
by  the  Westinghouse  Electric  and  Manufacturing  Company  and 
others  and  a  few  pounds  of  highly  impure  pyrophoric  powder 
made    by   Metal   Hydrides    Company.    The    only   considerable 
amount  of  raw  material  then  available  in  this  country  was  in  the 
form  of  a  commercial  grade  of  black  uranium  oxide,  which  could 
be  obtained  in  limited  quantities  from  the  Canadian  Radium 
and  Uranium  Co.  It  contained  2  to  5  per  cent  of  impurities  and 
was  the  material  which  gave  a  neutron  multiplication  factor  of 
only  about  0.87  when  used  in  an  exponential  pile. 

6.11.  By  May  1942,  deliveries  averaging  15  tons  a  month  of 
black  oxide  of  higher  purity  and  more  uniform  grade  started 
coming  in.  Total  impurities  were  less  than  1  per  cent,  boron 
comprised  a  few  parts  per  million,  and  the  neutron  multiplication 
factor  (k)  was  about  0.98.   (It  is  to  be  remembered  that  the 
multiplication  factor  depends  also  on  the  purity  of  the  graphite.) 
Deliveries  of  this  material  reached  a  ton  a  day  in  September  1942. 

Metallurgical  Project  in  1942  93 

6.12.  Experiments  at  the  National  Bureau  of  Standards  by 
J.  I.  Hoffman  demonstrated  that,  by  the  use  of  an  ether  extraction 
method,  all  the  impurities  are  removed  by  a  single  extraction  of 
uranyl  nitrate.  The  use  of  this  method  removed  the  great  bulk 
of  the  difficulties  in  securing  pure  oxide  and  pure  materials  for 
the  production  of  metal.  Early  in  May  1942,  arrangements  were 
completed  with  the  Mallinckrodt  Chemical  Works  in  St.  Louis 
to  put  the  new  grade  of  oxide  through  an  ether  extraction  process 
on  a  production  basis  for  a  further  reduction  in  impurity  content 
and  to  deliver  the  final  product  as  brown  dioxide.  Deliveries 
started  in  July  1942  at  a  rate  of  30  tons  a  month.  This  oxide  is 
now  used  as  a  starting  point  for  all  metal  production,  and  no 
higher  degree  of  purity  can  be  expected  on  a  commercial  scale. 
In  fact,  it  was  a  remarkable  achievement  to  have  developed  and 
put  into  production  on  a  scale  of  the  order  of  one  ton  per  day  a 
process  for  transforming  grossly  impure  commercial  oxide  to 
oxide  of  a  degree  of  purity  seldom  achieved  even  on  a  laboratory 

6.13.  The  process  which  Westinghouse  had  been  using  to  pro- 
duce the  metal  was  the  electrolysis  of  KUFs  at  a  cost  of  about 
$1,000  a  pound.  Since  the  KUFs  was  produced  photochemically 
under  the  action  of  sunlight  this  method  constituted  a  potential 
bottleneck  in  production.  It  was  found  that  uranium  tetrafluoride 
could  be  used  instead  of  KUFs,  and  steps  were  taken  to  have  this 
salt  produced  at  the  Harshaw  Chemical  Company  in  Cleveland 
and  at  the  du  Pont  plant  in  Penns  Grove,  New  Jersey.  Production 
started  in  August  1942  and  by  October  1942  was  up  to  700 
pounds  per  day  at  Harshaw  and  300  pounds  per  day  at  du  Pont, 
the  method  of  manufacture  in  both  cases  being  the  hydrofluorina- 
tion  of  Mallinckrodt-purified  dioxide. 

6.14.  As  the  result  of  this  supply  of  raw  materials  to  Westing- 
house,  and  as  a  result  of  plant  expansion,  deliveries  from  Westing- 
house  had  accumulated  to  a  total  of  more  than  6,000  pounds  by 
November  1 942  and  were  expected  to  be  at  the  rate  of  500  pounds 
per  day  by  January  1943.  The  purity  of  the  metal  was  good,  and 
the  cost  had  dropped  to  $22  per  pound. 

94  Metallurgical  Project  in  1942 

6.15.  Deliveries  of  acceptable  metal  from  Metal  Hydrides  Co. 
were  delayed  for  various  reasons  and  were  just  beginning  in 
November  1942.  This  company's  production  was  supposed  to 
reach  a  thousand  pounds  per  week  thereafter. 

6.16.  Neither  the  Westinghouse  process  nor  the  Metal  Hy- 
drides Process  was  entirely  satisfactory.  Intensive  activity  designed 
to  accelerate  metal  production,  and  carried  out  independently 
by  F.  H.  Spedding  and  his  associates  at  Iowa  State  College  at 
Ames,  Iowa,  and  by  C.  J.  Rodden  at  the  National  Bureau  of 
Standards,  resulted  in  the  development  of  a  satisfactory  method. 
Production  facilities  were  set  up  at  Ames  in  the  fall  of  1942  and 
had  already  produced  more  than  one  ton  by  the  end  of  November. 
The  process  was  extremely  simple,  rapid  and  low  cost. 

6.17.  Further  research  indicated  additional  changes  that  could 
be  made  to  advantage,  and  by  the  middle  of  1943  Spedding  at 
Iowa  and  other  producers  who  entered  the  picture  were  using 
the  final  production  method  adopted. 

6.18.  By  the  end  of  1942  arrangements  had  been  made  by  the 
Manhattan  District  to  increase  metal  production  by  making 
greater  use  of  the  Mallinckrodt  Chemical  Works,   the  Union 
Carbide  and  Carbon  Corporation,  and  the  du  Pont  Company. 

6.19.  To  summarize,  almost  no  metal  was  available  during 
most  of  1942,  a  fact  that  seriously  delayed  progress  as  we  shall 
see,  but  the  production  problems  had  been  nearly  solved  by  the 
end  of  1 942  and  some  6  tons  of  metal  were  incorporated  in  the 
pile  built  in  November  1942.  The  whole  problem  of  procurement 
of  metal  was  taken  over  by  the  Manhattan  District  at  the  end 
of  the  year,   under  the  general  direction  of  Colonel  RuhofT, 
formerly  with  the  Mallinckrodt  Chemical  Works.  From  the  point 
of  view  of  the  Metallurgical  Project  no  further  serious  delays  or 
difficulty  have  occurred  because  of  metal  shortages. 


6.20.  At  the  beginning  of  1942  graphite  production  was  still 
unsatisfactory  but  it  was,  of  course,  in  quite  a  different  condition 
from  the  metal  production  since  the  industrial  production  of 

Metallurgical  Project  in  1942  95 

graphite  had  already  been  very  large.  The  problem  was  merely 
one  of  purity  and  priority.  Largely  through  the  efforts  of  N.  Hil- 
berry,  the  National  Carbon  Company  and  the  Speer  Carbon 
Company  were  both  drawn  into  the  picture.  Following  sugges- 
tions made  by  the  experts  of  the  National  Bureau  of  Standards, 
these  companies  were  able  to  produce  highly  purified  graphite 
with  a  neutron  absorption  some  20  per  cent  less  than  the  standard 
commercial  materials  previously  used.  Although  efforts  further  to 
reduce  the  impurities  have  had  some  success,  the  purity  problem 
was  essentially  solved  by  the  middle  of  1942  and  large  orders 
were  placed  with  the  cooperation  of  the  War  Production  Board. 
As  in  the  case  of  the  metal,  the  graphite  procurement  problem 
was  taken  over  by  the  Manhattan  District. 



6.21.  At  the  time  that  the  Metallurgical  Project  was  organized, 
most  of  the  physicists  familiar  with  the  problem  believed  that  a 
chain-reacting  pile  probably  could  be  built  if  sufficiently  pure 
graphite  and  pure  uranium  metal  could  be  obtained.  Enough 
work  had  been  done  on  resonance  absorption,  on  the  theory  of 
absorption  and  diffusion  of  neutrons  hi  a  pile,  and  on  inter- 
mediate experiments  to  make  it  possible  to  design  a  lattice  struc- 
ture that  had  a  very  good  chance  of  maintaining  a  chain  reaction. 
Nevertheless,  there  were  uncertainties  in  the  experimental  data 
and  in  the  approximations  that  had  to  be  made  in  the  theoretical 
calculations.  There  were  two  alternatives:  (1)  to  build  a  pile 
according  to  the  best  possible  design;  (2)  to  make  more  accurate 
determinations  of  the  pertinent  nuclear  constants,  to  perform 
intermediate  experiments,  and  to  improve  the  calculations.  There 
is  little  doubt  that  the  first  alternative  was  the  one  likely  to  lead 
most  rapidly  to  the  production  of  plutonium.  There  were  many 
important  questions  which  could  have  been  answered  more 
rapidly  by  such  an  operating  pile  than  by  a  series  of  small-scale 
experiments.  Unfortunately,  the  necessary  amounts  of  materials 

96  Metallurgical  Project  in  1Q42 

were  not  available  and  did  not  become  available  for  nearly  nine 
months.  Consequently,  it  was  necessary  to  choose  the  second 
alternative,  that  is,  to  accumulate  all  relevant  or  possibly  relevant 
information  by  whatever  means  were  available. 

6.22.  The  major  line  of  investigation  was  a  series  of  inter- 
mediate experiments.  The  particular  set-up  for  each  intermediate 
experiment  could  be  used  to  test  calculations  based  on  separate 
auxiliary  experiments.  For  example,  the  proportion  of  uranium 
oxide  to  graphite  was  varied,  oxides  of  different  purities  were 
used,  oxide  was  used  in  lumps  of  various  sizes  and  shapes  and 
degrees  of  compression,  the  lattice  spacing  was  varied,  the  effect 
of  surrounding  the  uranium  oxide  units  with  beryllium  and  with 
paraffin  was  tried;  and,  finally,  piles  of  identical  lattice  type  but 
of  different  total  size  were  tried  to  see  whether  the  values  of  the 
multiplication  factor  k  (for  infinite  size)  calculated  from  the 
different  sets  of  results  were  identical.  In  general,  E.  Fermi  had 
direct  charge  of  investigations  of  effects  of  impurities,  and  S.  K. 
Allison  had  charge  of  tests  involving  different  lattice  dimensions. 
All  these  experiments  strengthened  the  confidence  of  the  group 
in  the  calculated  value  of  k  and  in  the  belief  that  a  pile  could  be 
built  with  k  greater  than  unity.  In  July  enough  purified  uranium 
oxide  from  Mallinckrodt  was  available  to  permit  building  inter- 
mediate pile  No.  9.  As  in  previous  experiments,  a  radium- 
beryllium  neutron  source  was  placed  at  the  bottom  of  the  lattice 
structure  and  the  neutron  density  measured  along  the  vertical 
axis  of  the  pile.  By  this  time  it  was  known  that  the  neutron  density 
decreased  exponentially  with  increasing  distance  from  the  neutron 
source  (hence  the  name  often  used  for  experiments  of  this  type, 
"exponential  pile")  and  that,  from  such  rates  of  decrease,  the 
multiplication  constant  k  for  an  infinitely  large  pile  of  the  same 
lattice  proportions  could  be  calculated.  For  the  first  time  the 
multiplication  constant  k  so  calculated  from  experimental  results 
came  out  greater  than  one.  (The  actual  value  was  1.007.)  Even 
before  this  experiment  Compton  predicted  in  his  report  of  July  1 
that  a  k  value  somewhere  between  1.04  and  1.05  could  be  ob- 
tained in  a  pile  containing  highly  purified  uranium  oxide  and 

Metallurgical  Project  in  1942  97 

graphite,  provided  that  the  air  was  removed  from  the  pile  to 
avoid  neutron  absorption  by  nitrogen. 


6.23.  We  shall  not  mention  a  majority  of  the  various  auxiliary 
experiments  done  during  this  period.  There  was  one,  however, — 
the  study  of  delayed  neutrons — that  we  shall  discuss  because  it  is 
a  good  example  of  the  kind  of  experiment  that  had  to  be  per- 
formed and  because  it  concerned  one  effect,  not  heretofore  men- 
tioned, that  is  of  great  importance  in  controlling  a  chain-reacting 

6.24.  From  previous  investigations,  some  of  which  were  already 
published,  it  was  known  that  about  1  per  cent  of  the  neutrons 
emitted  in  fission  processes  were  not  ejected  immediately  but 
were  given  off  in  decreasing  quantity  over  a  period  of  time,  a 
fact  reminiscent  of  the  emission  of  beta  rays  from  shortlived 
radioactive  substances.  Several  half-lives  had  been  observed,  the 
longest  being  of  the  order  of  a  minute. 

6.25.  It  was  realized  early  that  this  time  delay  gave  a  sort  of 
inertia  to  the  chain  reaction  that  should  greatly  facilitate  control. 
If  the  effective  multiplication  factor  of  a  pile  became  slightly 
greater  than  1,  the  neutron  density  would  not  rise  to  harmfully 
large  values  almost  instantly  but  would  rise  gradually  so  that 
there  would  be  a  chance  for  controls  to  operate.  (Other  time 
intervals  involved,  such  as  those  between  collisions,  are  too  small 
to  be  useful.) 

6.26.  Because  of  the  importance  of  this  effect  of  delayed  neu- 
trons for  control  it  was  decided  to  repeat  and  improve  the  earlier 
measurements.  (The  fact  that  this  was  a  repetition  rather  than  a 
new  measurement  is  also  typical  of  much  of  the  work  in  physics 
at  this  period.)  A  description  of  the  experiment  is  given  in  Ap- 
pendix 3.  The  results  indicated  that  1.0  per  cent  of  the  neutrons 
emitted  in  uranium  fission  are  delayed  by  at  least  0.01  second 
and  that  about  0.7  per  cent  are  delayed  by  as  much  as  a  minute. 
By  designing  a  pile  such  that  the  effective  value  of  k,  the  multi- 

98  Metallurgical  Project  in  1942 

plication  factor,  is  only  1.01  the  number  of  delayed  neutrons  is 
sufficient  to  allow  easy  control. 


6.27.  By  the  fall  of  1942  enough  graphite,  uranium  oxide,  and 
uranium  metal  were  available  at  Chicago  to  justify  an  attempt 
to  build  an  actual  self-sustaining  chain-reacting  pile.  But  the 
amount  of  metal  available  was  small — only  about  6  tons — and 
other  materials  were  none  too  plentiful  and  of  varying  quality. 
These  conditions  rather  than  optimum  efficiency  controlled  the 

6.28.  The  pile  was  constructed  on  the  lattice  principle  with 
graphite  as  a  moderator  and  lumps  of  metal  or  oxide  as  the 
reacting  units  regularly  spaced  through  the  graphite  to  form  the 
lattice.  Instruments  situated  at  various  points  in  the  pile  or  near 
it  indicated  the  neutron  intensity,  and  movable  strips  of  absorbing 
material  served  as  controls.  (For  a  more  complete  description  of 
the  pile,  see  Appendix  4.)  Since  there  were  bound  to  be  some 
neutrons  present  from  spontaneous  fission  or  other  sources,  it 
was  anticipated  that  the  reaction  would  start  as  soon  as  the  struc- 
ture had  reached  critical  size  if  the  control  strips  were  not  set 
in  "retard"  position.  Consequently,  the  control  strips  were  placed 
in  a  suitable  "retard"  position  from  the  start  and  the  neutron 
intensity  was  measured  frequently.  This  was  fortunate  since  the 
approach  to  critical  condition  was  found  to  occur  at  an  earlier 
stage  of  assembly  than  had  been  anticipated. 

6.29.  The  pile  was  first  operated  as  a  self-sustaining  system  on 
December  2,  1942.  So  far  as  we  know,  this  was  the  first  time  that 
human  beings  ever  initiated  a  self-maintaining  nuclear  chain 
reaction.   Initially  the  pile  was  operated  at  a  power  level  of 
J^  watt,  but  on  December  12  the  power  level  was  raised  to  200 


6.30.  In  these  experiments  no  direct  measurements  of  energy 
release  were  made.  The  number  of  neutrons  per  second  emitted 

Metallurgical  Project  in  1942  99 

by  the  pile  was  estimated  in  terms  of  the  activity  of  standardized 
indium  foils.  Then,  from  a  knowledge  of  the  number  of  neutrons 
produced  per  fission,  the  resultant  rate  of  energy  release  (wattage) 
was  calculated. 


6.31.  Evidently  this  experiment,  performed  on  December  2 
just  as  a  reviewing  committee  was  appraising  the  Chicago  project, 
answered  beyond  all  shadow  of  doubt  the  first  question  before  the 
Metallurgical  Laboratory;  a  self-sustaining  nuclear  chain  reac- 
tion had  been  produced  in  a  system  using  normal  uranium.  This 
experiment  had  been  performed  under  the  general  direction  of 
E.  Fermi,  assisted  principally  by  the  groups  headed  by  W.  H.  Zinn 
and  H.  L.  Anderson.  V.  C.  Wilson  and  his  group  had  been  largely 
responsible  for  developing  the  instruments  and  controls,  and  a 
great  many  others  in  the  laboratory  had  contributed  to  the 
success  of  the  enterprise. 


6.32.  The  immediate  object  of  building  a  uranium-graphite 
pile  was  to  prove  that  there  were  conditions  under  which  a  chain 
reaction  would  occur,  but  the  ultimate  objective  of  the  laboratory 
was  to  produce  plutonium  by  a  chain  reaction.  Therefore  we  are 
interested  in  the  relation  between  the  power  at  which  a  pile 
operates  and  the  rate  at  which  it  produces  plutonium.  The  rela- 
tion may  be  evaluated  to  a  first  approximation  rather  easily. 
A  pile  running  stably  must  be  producing  as  many  neutrons  as  it 
is  losing.  For  every  thermal  neutron  absorbed  in  U-235  a  certain 
number  of  neutrons,  r;,  is  emitted.  One  of  these  neutrons  is  re- 
quired to  maintain  the  chain.  Therefore,  assuming  the  extra 
neutrons  all  are  absorbed  by  U-238  to  form  plutonium,  there  will 
be  TJ  —  1  atoms  of  Pu239  formed  for  every  fission.  Every  fission 
releases  roughly  200  Mev  of  energy.  Therefore  the  formation 
of  77  —  1  atoms  of  plutonium  accompanies  the  release  of  about 
200  Mev.  Since  77  —  1  is  a  small  number,  we  can  guess  that  to 

100  Metallurgical  Project  in  1942 

produce  a  kilogram  a  day  of  plutonium  a  chain-reacting  pile 
must  be  releasing  energy  at  the  rate  of  500,000  to  1,500,000  kilo- 
watts. The  first  chain-reacting  pile  that  we  have  just  described 
operated  at  a  maximum  of  200  watts.  Assuming  that  a  single 
bomb  will  require  the  order  of  one  to  100  kilograms  of  plutonium, 
the  pile  that  has  been  described  would  have  to  be  kept  going  at 
least  70,000  years  to  produce  a  single  bomb.  Evidently  the  prob- 
lem of  quantity  production  of  plutonium  was  not  yet  solved. 


6.33.  The  second  specific  objective  of  the  Metallurgical 
Laboratory  was  to  show  that,  if  a  chain  reaction  did  occur,  it 
would  be  feasible  to  separate  the  plutonium  chemically  from  the 
other  material  with  which  it  is  found.  Progress  toward  this  objec- 
tive was  necessarily  slower  than  toward  the  attainment  of  a  chain 
reaction.  Initially  little  was  done  at  the  Metallurgical  Laboratory 
on  chemical  problems  although  the  extraction  problem  was  dis- 
cussed in  a  conference  soon  after  the  project  was  organized  and 
the  work  of  Seaborg's  group  at  the  University  of  California  on 
plutonium  was  encouraged.  On  April  22-23,  1942,  a  general 
conference  on  chemistry  was  held  at  Chicago,  attended  by  F.  H. 
Spedding,  E.  W.  Thiele,  G.  T.  Seaborg,  J.  W.  Kennedy,  H.  C. 
Urey,  E.  Wigner,  N.  Hilberry,  G.  E.  Boyd,  I.  B.Johns,  H.  A.  Wil- 
helm,  I.  Perlman,  A.  C.  Wahl,  and  J.  A.  Wheeler.  Spedding,  in 
opening  the  meeting,  pointed  out  that  there  were  two  main  tasks 
for  the  chemists:  first,  to  separate  plutonium  in  the  amounts  and 
purity  required  for  war  purposes;  second,  to  obtain  a  good  under- 
standing of  the  chemistry  necessary  for  the  construction  and 
maintenance  of  the  pile.  The  separation  problem  was  to  be  studied 
by  a  new  group  at  Chicago  under  the  direction  of  Seaborg,  by 
Johns  and  Wilhelm  at  Ames,  and  by  Wahl  and  Kennedy  con- 
tinuing the  work  at  California.  Other  closely  related  groups  at 
Chicago  were  to  be  C.  D.  Coryell's,  working  on  the  fission  prod- 
ucts, and  Boyd's  on  analytical  problems.  The  chemistry  group  at 
Chicago  has  grown  speedily  since  that  time.  A  new  building  had 
to  be  constructed  to  house  it  late  in  1942,  and  this  building  was 

Metallurgical  Project  in  1942  101 

enlarged  subsequently.  Altogether,  the  solving  of  many  of  the 
chemical  problems  has  been  one  of  the  most  remarkable  achieve- 
ments of  the  Metallurgical  Laboratory. 

6.34.  The  first  isotope  of  plutonium  discovered  and  studied 
was  not  the  239  isotope  but  the  238  isotope,  which  is  an  alpha-ray 
emitter  with  a  half-life  of  about  50  years.  U-238  bombarded  with 
deuterons  gives  gsNp238  which  disintegrates  to  94Pu238  by  beta 
emission.*  The  first  evidence  of  the  actual  existence  of  these  new 
elements  (ruling  out  the  original  erroneous  interpretation  of  the 
splitting  of  uranium  as  evidence  for  their  existence)  was  obtained 
by  E.  McMillan  and  P.  H.  Abelson  who  isolated  93-238  from 
uranium  bombarded  with  deuterons  in  the  Berkeley  cyclotron. 
This  new  element  wras  identified  as  a  beta  emitter  but  the  sample 
was  too  small  for  isolation  of  the  daughter  product  94-238.  Later, 
enough  Pu-238  was  prepared  to  permit  Seaborg,  Kennedy  and 
\\  ahl  to  begin  the  study  of  its  chemical  properties  in  the  winter 
of  1940-1941  by  using  tracer  chemistry  with  carriers  according 
to    practice    usual    in    radiochemistry.    By   such  studies   many 
chemical  properties  of  plutonium  were  determined,  and  several 
possible  chemical  processes  were  evolved  by  which  Pu-239  might 
be  removed  from  the  chain-reacting  pile.  The  success  of  experi- 
ments on  a  tracer  scale  led  to  plans  to  produce  enough  Pu-239 
to  be  treated  as  an  ordinary  substance  on  the  ultra-microchemical 
M-ale.  Such  quantities  were  produced  by  prolonged  bombard- 
ment of  several  hundred  pounds  of  uranyl  nitrate  with  neutrons 
obtained  with  the  aid  of  cyclotrons,  first  at  Berkeley  and  later  at 
Washington  University  in  St.  Louis.  By  the  end  of  1942,  some- 
tiling  over  500  micrograms  had  been  obtained  in  the  form  of  pure 
plutonium  salts.  Although  this  amount  is  less  than  would  be 
needed  to  make  the  head  of  a  pin,  for  the  micro-chemists  it  was 
sufficient  to  yield  considerable  information;  for  one  microgram 
is    considered    sufficient    to    carry    out    weighing   experiments, 
dtrations,  solubility  studies,  etc. 

6.35.  From  its  position  in  the  periodic  table,  plutonium  might 
l;>e  expected  to  be  similar  to  the  rare  earths  or  to  uranium* 

*  See  drawing  on  p.  8. 

102  Metallurgical  Project  in  1942 

thorium,  or  osmium.  Which  of  these  it  will  resemble  most  closely 
depends,  of  course,  on  the  arrangement  of  the  outermost  groups 
of  electrons  and  this  arrangement  could  hardly  have  been  pre- 
dicted. On  the  whole,  plutonium  turned  out  to  be  more  like 
uranium  than  like  any  of  the  other  elements  named  and  might 
even  be  regarded  as  the  second  member  of  a  new  rare-earth 
series  beginning  with  uranium.  It  was  discovered  fairly  early 
that  there  were  at  least  two  states  of  oxidation  of  plutonium. 
(It  is  now  known  that  there  are  four,  corresponding  to  positive 
valences  of  3,  4,  5,  and  6.)  Successful  microchemical  preparation 
of  some  plutonium  salts  and  a  study  of  their  properties  led  to  the 
general  conclusion  that  it  was  possible  to  separate  plutonium 
chemically  from  the  other  materials  in  the  pile.  This  conclusion 
represents  the  attainment  of  the  second  immediate  objective  of 
the  Metallurgical  Laboratory.  Thus,  by  the  end  of  1942, 
plutonium,  entirely  unknown  eighteen  months  earlier,  was  con- 
sidered an  element  whose  chemical  behavior  was  as  well  under- 
stood as  that  of  several  of  the  elements  of  the  old  periodic  table. 


6.36.  Besides  the  major  problems  we  have  mentioned,  i.e.,  the 
chain  reaction,  the  chemical  separation,  and  the  planning  for  a 
production  plant,  there  were  innumerable  minor  problems  to  be 
solved.  Among  the  more  important  of  these  were  the  improve- 
ment of  neutron  counters,  ionization  chambers,  and  other  instru- 
ments, the  study  of  corrosion  of  uranium  and  aluminum  by 
water  and  other  possible  coolants,  the  determination  of  the 
effects  of  temperature  variation  on  neutron  cross  sections,  the 
fabrication  of  uranium  rods  and  tubes,  the  study  of  fission  prod- 
ucts, and  the  determination  of  the  biological  effects  of  radiation. 
As  typical  of  this  kind  of  work  we  can  cite  the  development  of 
methods  of  fabricating  and  coating  uranium  metal,  under  the 
direction  of  E.  Creutz.  Without  the  accomplishment  of  these 
secondary  investigations  the  project  could  not  have  reached  its 
goal.  To  give  some  further  idea  of  the  scope  of  the  work,  a  list 

Metallurgical  Project  in  1942  103 

of  twenty  report  titles  is  presented  in  Appendix  5,  the  20  reports 
being  selected  from  the  400  or  so  issued  during  1942. 


6.37.  The  third  initial  objective  of  the  Metallurgical  Project 
was   to  obtain   theoretical  and  experimental  data  on  a  "fast 
neutron"  reaction,  such  as  would  be  required  in  an  atomic 
bomb.  This  aspect  of  the  work  was  initially  planned  and  coordi- 
nated by  G.  Breit  of  the  University  of  Wisconsin  and  later  con- 
tinued by  J.  R.  Oppenheimer  of  the  University  of  California. 
Since  the  actual  construction  of  the  bomb  was  to  be  the  final  part 
of  the  program,  the  urgency  of  studying  such  reactions  was  not 
so  great.  Consequently,  little  attention  was  given  to  the  theo- 
retical problems  until  the  summer  of  1942,  when  a  group  was 
organized  at  Chicago  under  the  leadership  of  Oppenheimer. 

6.38.  In  the  meantime  experimental  work  initiated  in  most 
instances  by  G.  Breit,  had  been  in  progress  (under  the  general 
direction  of  the  Metallurgical  Project)   at  various  institutions 
having  equipment  suitable  for  fast-neutron  studies    (Carnegie 
Institution  of  Washington,  the  National  Bureau  of  Standards, 
Cornell  University,  Purdue  University,  University  of  Chicago, 
University  of  Minnesota,  University  of  Wisconsin,  University  of 
California,  Stanford  University,  University  of  Indiana,  and  Rice 
Institute).  The  problems  under  investigation  involved  scattering, 
absorption   and   fission   cross  section,   the   energy  spectrum   of 
fission  neutrons,  and  the  time  delay  in  the  emission  of  fission 
neutrons.  For  the  most  part  this  work  represented  an  intermediate 
step  in  confirming  and  extending  previous  measurements  but 
reached  no  new  final  conclusion.  This  type  of  work  was  subse- 
quently concentrated  at  another  site  (see  Chapter  XII). 

6.39.  As  indicated  by  the  "Feasibility  Report"  (in  a  section 
written  by  J.  H.  Manley,  J.  R.  Oppenheimer,  R.  Serber,  and 
E.  Teller)   the  picture  had  changed  significantly  in  only  one 
respect  since  the  appearance  of  the  National  Academy  Report 
a  year  earlier.  Theoretical  studies  now  showed  that  the  effective- 
ness of  the  atomic  bomb  in  producing  damage  would  be  greater 

104  Metallurgical  Project  in  1942 

than  had  been  indicated  in  the  National  Academy  report.  How- 
ever, critical  size  of  the  bomb  was  still  unknown.  Methods  of 
detonating  the  bomb  had  been  investigated  somewhat,  but  on 
the  whole  no  certain  answers  had  been  reached. 




6.40.  As  we  have  seen,  the  initial  objectives  of  the  Metallurgical 
Laboratory  had  been  reached  by  the  end  of  1942,  but  the  ulti- 
mate objectives,  the  production  of  large  quantities  of  plutonium 
and  the  design  and  fabrication  of  bombs,  were  still  far  from  at- 
tained. The  responsibility  for  the  design  and  fabrication  of  bombs 
was  transferred  to  another  group  at  about  this  time;  its  work  is 
reported  in  Chapter  XII.  The  production  of  Pu-239  in  quantity 
has  remained  the  principal  responsibility  of  the  Metallurgical 
Laboratory  although  shared  with  the  du  Pont  Company  since 
the  end  of  1942. 

6.41 .  On  the  basis  of  the  evidence  available  it  was  clear  that 
a  plutonium  production  rate  somewhere  between  a  kilogram  a 
month  and  a  kilogram  a  day  would  be  required.  At  the  rate  of  a 
kilogram  a  day.  a  500,000  to  1,500,000  kilowatt  plant  would  be 
required.    (The  ultimate   capacity  of  the   hydroelectric  power 
plants  at  the  Grand  Coulee  Dam  is  expected  to  be  2,000,000  kw.) 
Evidently  the  creation  of  a  plutonium  production  plant  of  the 
required  size  was  to  be  a  major  enterprise  even  without  attempt- 
ing to  utilize  the   thermal  energy  liberated.   Nevertheless,   by 
November  1942  most  of  the  problems  had  been  well  defined  and 
tentative  solutions  had  been  proposed.  Although  these  problems 
will  be  discussed  in  some  detail  in  the  next  chapter,  we  will 
mention  them  here. 

6.42.  Since  a  large  amount  of  heat  is  generated  in  any  pile 
producing  appreciable  amounts  of  plutonium,  the  first  problem 

Metallurgical  Project  in  1942  105 

of  design  is  a  cooling  system.  Before  such  a  system  can  be  de- 
signed, it  is  necessary  to  find  the  maximum  temperature  at  which 
a  pile  can  run  safely  and  the  factors — nuclear  or  structural — 
\\  hich  determine  this  temperature.  Another  major  problem  is  the 
method  for  loading  and  unloading  the  uranium,  a  problem  com- 
plicated by  the  shielding  and  the  cooling  system.  Shielding  against 
radiation  has  to  be  planned  for  both  the  pile  itself  and  the  chemi- 
cal separation  plant.  The  nature  of  the  separation  plant  depends 
on  the  particular  separation  process  to  be  used,  which  has  to  be 
decided.  Finally,  speed  of  procurement  and  construction  must 
be  primary  factors  in  the  planning  of  both  the  pile  and  the 
chemical  plant. 


6.43.  After  examining  the  principal  factors  affecting  plant 
design,  i.e.,  cooling,  efficiency,  safety,  and  speed  of  construction, 
the  "Feasibility  Report"  suggested  a  number  of  possible  plant 
types  in  the  following  order  of  preference: 

I.  (a)  Ordinary  uranium  metal  lattice  in  a  graphite  moder- 
ator  with  helium  cooling,    (b)   The  same,   with  water 
cooling,  (c)  The  same,  with  molten  bismuth  cooling. 
II.  Ordinary  uranium  metal  lattice  in  a  heavy-water  moder- 

III.  Uranium  enriched  in  the   235   isotope  using  graphite, 
heavy  water,  or  ordinary  water  as  moderator. 

Types  II  and  III  were  of  no  immediate  interest  since  neither 
enriched  uranium  nor  heavy  water  was  available.  Development 
of  both  these  types  continued  however,  since  if  no  other  type 
proved  feasible  they  might  have  to  be  used.  Type  I  (c),  calling 
for  liquid  bismuth  cooling,  seemed  very  promising  from  the  point 
of  view  of  utilization  of  the  thermal  energy  released,  but  it  was 
felt  that  the  technical  problems  involved  could  not  be  solved  for 
a  long  time. 

106  Metallurgical  Project  in  1942 


6.44.  During  this  period,  the  latter  half  of  1942,  when  pro- 
duction plants  were  being  planned,  it  was  recognized  that  a  plant 
of  intermediate  size  was  desirable.  Such  a  plant  was  needed  for 
two  reasons:  first,  as  a  pilot  plant;  second,  as  a  producer  of  a  few 
grams  of  plutonium  badly  needed  for  experimental  purposes. 
Designed  as  an  air-cooled  plant  of  1,000-kw  capacity,  the  inter- 
mediate  pile   constructed   at   Clinton,   Tennessee,    might   have 
served  both  purposes  if  helium  cooling  had  been  retained  for  the 
main  plant.  Although  the  plans  for  the  main  plant  were  shifted 
so  that  water  cooling  was  called  for,  the  pilot  plant  was  continued 
with  air  cooling  in  the  belief  that  the  second  objective  would  be 
reached  more  quickly.  It  thus  ceased  to  be  a  pilot  plant  except 
for   chemical  separation.   Actually   the   main   plant  was   built 
without  benefit  of  a  true  pilot  plant,  much  as  if  the  hydroelectric 
generators  at  Grand  Coulee  had  been  designed  merely  from 
experience  gained  with  a  generator  of  quite  different  type  and 
of  a  small  fraction  of  the  power. 


6.45.  As  reviewed  by  Hilberry  in  the  "Feasibility  Report"  of 
November  26,  1 942,  the  prospects  for  a  graphite  pile  with  helium 
cooling  looked  promising  as  regards  immediate  production;  the 
pile  using  heavy  water  for  moderator  and  using  heavy  water  or 
ordinary  water  as  coolant  looked  better  for  eventual  full-scale 
use.  A  number  of  specific  proposals  were  made  for  construction 
of  such  plants  and  for  the  further  study  of  the  problems  involved. 
These  proposals  were  based  on  time  and  cost  estimates  which  were 
necessarily  little  better  than  rough  guesses.  As  the  result  of  further 
investigation  the  actual  program  of  construction — described  in 
later  chapters — has  been  quite  different  from  that  proposed. 


6.46.  The  procurement  problem  which  had   been  delaying 
progress  was  essentially  solved  by  the  end  of  1942.  A  small  self- 

Metallurgical  Project  in  1942  107 

sustaining  graphite-uranium  pile  was  constructed  in  November 
1942,  and  was  put  into  operation  for  the  first  time  on  December  2, 
1942,  at  a  power  level  of  }/£  watt  and  later  at  200  watts.  It  was 
easily  controllable  thanks  to  the  phenomenon  of  delayed  neutron 
emission.  A  total  of  500  micrograms  of  plutonium  was  made  with 
the  cyclotron  and  separated  chemically  from  the  uranium  and 
fission  products.  Enough  was  learned  of  the  chemistry  of  plu- 
tonium to  indicate  the  possibility  of  separation  on  a  relatively 
large  scale.  No  great  advance  was  made  on  bomb  theory,  but 
calculations  were  checked  and  experiments  with  fast  neutrons 
extended.  If  anything,  the  bomb  prospects  looked  more  favorable 
than  a  year  earlier. 

6.47.  Enough  experimenting  and  planning  were  done  to 
delineate  the  problems  to  be  encountered  in  constructing  and 
operating  a  large-scale  production  plant.  Some  progress  was 
made  in  choice  of  type  of  plant,  first  choice  at  that  time  being  a 
pile  of  metallic  uranium  and  graphite,  cooled  either  by  helium 
or  water.  A  specific  program  was  drawn  up  for  the  construction 
of  pilot  and  production  plants.  This  program  presented  time  and 
cost  estimates. 




7.1.  By  the  first  of  January  1943,  the  Metallurgical  Laboratory 
had  achieved  its  first  objective,  a  chain-reacting  pile,  and  was 
well  on   the  way  to  the  second,   a  process  for  extracting  the 
plutonium  produced  in  such  a  pile.  It  was  clearly  time  to  formu- 
late more  definite  plans  for  a  production  plant.  The  policy  de- 
cisions were  made  by  the  Policy  Committee  (see  Chapter  V)  on 
the   recommendations   from   the   Laboratory   Director    (A.    H. 
Compton),  from  the  S-l   Executive  Committee,  and  from  the 
Reviewing  Committee  that  had  visited  Chicago  in  December 
1942.  The  only  decisions  that  had  already  been  made  were  that 
the   first   chain-reacting   pile   should   be   dismantled    and   then 
reconstructed  on  a  site  a  short  distance  from  Chicago  and  that 
a  1,000-kilowatt  plutonium  plant  should  be  built  at  Clinton, 


7.2.  The  first  decision  to  be  made  was  on  the  scale  of  pro- 
duction that  should  be  attempted.  For  reasons  of  security  the 
figure  decided  upon  may  not  be  disclosed  here.  It  was  very  large. 


7.3.  As  we  have  seen,  the  production  of  one  gram  of  plutonium 
per  day  corresponds  to  a  generation  of  energy  at  the   rate   of 
500  to  1,500  kilowatts.  Therefore  a  plant  for  large-scale  produc- 
tion of  plutonium  will  release  a  very  large  amount  of  energy. 


Plutonium  Problem  February  1943  109 

The  problem  therefore  was  to  design  a  plant  of  this  capacity  on 
the  basis  of  experience  with  a  pile  that  could  operate  at  a  power 
level  of  only  0.2  kilowatt.  As  regards  the  plutonium  separation 
work,  which  was  equally  important,  it  was  necessary  to  draw 
plans  for  an  extraction  and  purification  plant  which  would 
separate  some  grams  a  day  of  plutonium  from  some  tons  of 
uranium,  and  such  planning  had  to  be  based  on  information 
obtained  by  microchemical  studies  involving  only  half  a  milli- 
gram of  plutonium.  To  be  sure,  there  w*as  information  available 
for  the  design  of  the  large-scale  pile  and  separation  plant  from 
auxiliary  experiments  and  from  large-scale  studies  of  separation 
processes  using  uranium  as  a  stand-in  for  plutonium,  but  even 
so  the  proposed  extrapolations  both  as  to  chain-reacting  piles 
and  as  to  separation  processes  were  staggering.  In  peacetime  no 
engineer  or  scientist  in  his  right  mind  would  consider  making 
such  a  magnification  in  a  single  stage,  and  even  in  wartime  only 
the  possibility  of  achieving  tremendously  important  results 
could  justify  it. 


7.4.  As  soon  as  it  had  been  decided  to  go  ahead  with  large- 
scale  production  of  plutonium,  it  was  evident  that  a  great  ex- 
pansion in  organization  was  necessary.  The  Stone  and  Webster 
Engineering  Corporation  had  been  selected  as  the  overall  engi- 
neering and  construction  firm  for  the  DSM  Project  soon  after 
the  Manhattan  District  was  placed  in  charge  of  construction 
work  in  June  1942.  By  October  1942,  it  became  evident  that 
various  component  parts  of  the  work  were  too  far  separated 
physically  and  were  too  complicated  technically  to  be  handled 
by  a  single  company — especially  in  view  of  the  rapid  pace  re- 
quired. Therefore  it  was  decided  that  it  would  be  advantageous 
if  Stone  and  Webster  were  relieved  of  that  portion  of  the  work 
pertaining  to  the  construction  of  plutonium  production  facilities. 
This  was  done,  and  General  Groves  selected  the  E.  I.  du  Pont 
de  Nemours  and  Company  as  the  firm  best  able  to  carry  on  this 
phase  of  the  work.  The  arrangements  made  with  various  indus- 

110  Plutonium  Problem  February  1943 

trial  companies  by  the  Manhattan  District  took  various  forms. 
The  arrangements  with  du  Pont  are  discussed  in  detail  as  an 

7.5.  General  Groves  broached  the  question  to  W.  S.  Carpenter, 
Jr.,  president  of  du  Pont,  and  after  considerable  discussion  with 
him  and  other  officials  of  the  firm,  du  Pont  agreed  to  undertake 
the  work.  In  their  acceptance,  they  made  it  plain  and  it  was 
understood  by  all  concerned  that  du  Pont  was  undertaking  the 
work  only  because  the  War  Department  considered  the  work  to 
be  of  the  utmost  importance,  and  because  General  Groves  stated 
that  this  view  as  to  importance  was  one  held  personally  by  the 
President  of  the  United  States,  the  Secretary  of  War,  the  Chief 
of  Staff,  and  General  Groves,  and  because  of  General  Groves' 
assertion  that  du  Pont  was  by  far  the  organization  best  qualified 
for  the  job.  At  the  same  time,  it  was  recognized  that  the  du  Pont 
Company  already  had  assumed  all  the  war-connected  activities 
which  their  existing  organization  could  be  expected  to  handle 
without  undue  difficulty. 

7.6.  The  du  Pont  Company,  in  accepting  the  undertaking, 
insisted  that  the  work  be  conducted  without  profit  and  without 
patent  rights  of  any  kind  accruing  to  them.  The  du  Pont  Com- 
pany did  request,  however,  that  in  view  of  the  unknown  character 
of  the  field  into  which  they  were  being  asked  to  embark,  and  in 
view  of  the  unpredictable  hazards  involved,  the  Government 
provide  maximum  protection  against  losses  sustained  by  du  Pont. 

7.7.  The  cost-plus-a-fixed-fee  contract  between  the  Govern- 
ment and  du  Pont  established  a  fixed  fee  of  SI. 00.  The  Govern- 
ment agreed  to  pay  all  costs  of  the  work  by  direct  reimbursement 
or  through  allowances  provided  by  the  contract  to  cover  ad- 
ministrative and  general  expenses  allocated  to  the  work  in  accord- 
ance with  normal  du  Pont  accounting  practices  as  determined  by 
audit  by  certified  public  accountants.  Under  the  terms  of  the 
contract,  any  portion  of  these  allowances  not  actually  expended 
by  du  Pont  will,  at  the  conclusion  of  the  work,  be  returned  to  the 
United  States.  The  contract  also  provided  that  no  patent  rights 
would  accrue  to  the  company. 

Plutonium  Problem  February  1943  111 

7.8.  The  specific  responsibilities  assumed  by  du  Pont  were  to 
engineer,  design,  and  construct  a  small-scale  semi- works  at  the 
Clinton  Engineer  Works  in  Tennessee  and  to  engineer,  design, 
construct,  and  operate  a  large-scale  plutonium  production  plant 
of  large  capacity  at  the  Hanford  Engineer  Works  in  the  State 
of  Washington.  Because  of  its  close  connection  with  fundamental 
research,  the  Clinton  semi-works  was  to  be  operated  under  the 
direction  of  the  University  of  Chicago.  A  large  number  of  key 
technical  people  from  du  Pont  were  to  be  used  on  a  loan  basis  at 
Chicago  and  at  Clinton,  to  provide  the  University  with  much 
needed  personnel,  particularly  men  with  industrial  experience, 
and  to  train  certain  of  such  personnel  for  future  service  at  Hanford. 

7.9.  Inasmuch  as  du  Pont  was  being  asked  to  step  out  of  its 
normal  role  in  chemistry  into  a  new  field  involving  nuclear 
physics,  it  was  agreed  that  it  would  be  necessary  for  them  to 
depend  most  heavily  upon  the  Metallurgical  Laboratory  of  the 
University  of  Chicago  for  fundamental  research  and  development 
data  and  for  advice.  The  du  Pont  Company  had  engineering  and 
industrial  experience,  but  it  needed  the  Metallurgical  Laboratory 
for  nuclear-physics  and  radiochemistry  experience.  The  Metal- 
lurgical  Laboratory   conducted   the   fundamental   research   on 
problems  bearing  on  the  design  and  operation  of  the  semi-works 
and  large-scale  production  plants.  It  proposed  the  essential  parts 
of  the  plutonium  production  and  recovery  processes  and  equip- 
ment, answered  the  many  specific  questions  raised  by  du  Pont, 
and  studied  and  concurred  in  the  final  du  Pont  decisions  and 

7.10.  The  principal  purpose  of  the  Clinton  semi- works  was 
development  of  methods  of  operation  for  plutonium  recovery. 
The  semi-works  had  to  include  of  course,  a  unit  for  plutonium 
production,  in  order  to  provide  plutonium  to  be  recovered  experi- 
mentally. In  the  time  and  with  the  information  available,  the 
Clinton  production  unit  could  not  be  designed  to  be  an  early 
edition  of  the  Hanford  production  units  which,  therefore,  had  to 
be  designed,  constructed  and  operated  without  major  guidance 
from  Clinton  experience.   In  fact,  even  the  Hanford  recover}' 

112  Plutonium  Problem  February  1943 

units  had  to  be  far  along  in  design  and  procurement  of  equip- 
ment before  Clinton  results  became  available.  However,  the 
Clinton  semi-works  proved  to  be  an  extremely  important  tool  in 
the  solution  of  the  many  completely  new  problems  encountered 
at  Hanford.  It  also  produced  small  quantities  of  plutonium 
which,  along  with  Metallurgical  Laboratory  data  on  the  prop- 
erties of  plutonium,  enabled  research  in  the  use  of  this  material 
to  be  advanced  many  months. 


7.11.  Once  the  scale  of  production  had  been  agreed  upon 
and  the  responsibilities  assigned,  the  nature  of  the  plant  and  its 
whereabouts  had  to  be  decided.  The  site  in  the  Tennessee  Valley, 
known  officially  as  the  Clinton  Engineer  Works,  had  been  ac- 
quired by  the  Army  for  the  whole  program  as  recommended  in 
the  report  to  the  President  (see  Chapter  V). 

7.12.  Reconsideration  at  the  end  of  1942  led  General  Groves 
to  the  conclusion  that  this  site  was  not  sufficiently  isolated  for  a 
large-scale  plutonium  production  plant.  At  that  time,  it  was 
conceivable  that  conditions  might  arise  under  which  a  large  pile 
might  spread  radioactive  material  over  a  large  enough  area  to 
endanger  neighboring  centers  of  population.  In  addition  to  the 
requirement  of  isolation,  there  remained  the  requirement  of  a 
large  power  supply  which  had  originally  determined  the  choice 
of  the  Tennessee  site.  To  meet  these  two  requirements  a  new  site 
was  chosen  and  acquired  on  the  Columbia  River  in  the  central 
part  of  the  State  of  Washington  near  the  Grand  Coulee  power 
line.  This  site  was  known  as  the  Hanford  Engineer  Works. 

7.13.  Since  the  Columbia  River  is  the  finest  supply  of  pure 
cold  river  water  in  this  country,  the  Hanford  site  was  well  suited 
to  either  the  helium-cooled  plant  originally  planned  or  to  the 
water-cooled  plant  actually  erected.  The  great  distances  sepa- 
rating the  home  office  of  du  Pont  in  Wilmington,  Delaware,  the 
pilot  plant  at  Clinton,  Tennessee,  the  Metallurgical  Laboratory 
at  Chicago,  and  the  Hanford  site  were  extremely  inconvenient, 
but  this  separation  could  not  be  avoided.  Difficulties  also  were 

Plutonium  Problem  February  1943  113 

inherent  in  bringing  workmen  to  the  site  and  in  providing  living 
accommodations  for  them. 


7.14.  It  was  really  too  early  in  the  development  to  make  a 
carefully  weighed  decision  as  to  the  best  type  of  plutonium  pro- 
duction plant.  Yet  a  choice  had  to  be  made  so  that  design  could 
be  started  and  construction  begun  as  soon  as  possible.  Actually 
a  tentative  choice  was  made  and  then  changed. 

7.15.  In  November   1942,   the  helium-cooled  plant  was   the 
first  choice  of  the  Metallurgical  Laboratory.  Under  the  direction 
of  T.  Moore  and  M.  C.  Leverett,  preliminary  plans  for  such 
a  plant  had  been  worked  out.  The  associated  design  studies  were 
used  as  bases  for  choice  of  site,  choice  of  accessory  equipment, 
etc.  Although  these  studies  had  been  undertaken  partly  because 
it  had  been  felt  that  they  could  be  carried  through  more  quickly 
for  a  helium-cooled  plant  than  for  a  water-cooled  plant,  many 
difficulties  were  recognized.   Meanwhile  the  theoretical  group 
under  Wigner,  with  the  cooperation  of  the  engineering  personnel, 
had  been  asked  to  prepare  a  report  on  a  water-cooled  plant  of 
high  power  output.  This  group  had  been  interested  in  water- 
cooling  almost  from  the  beginning  of  the  project  and  was  able  to 
incorporate  the  results  of  its  studies  in  a  report  issued  on  January 
9,  1943.  This  report  contained  many  important  ideas  that  were 
incorporated  in  the  design  of  the  production  plant  erected  at 

7.16.  When  du  Pont  came  into  the  picture,  it  at  first  accepted 
the  proposal  of  a  helium-cooled  plant  but  after  further  study 
decided  in  favor  of  water  cooling.  The  reasons  for  the  change 
were  numerous.  Those  most  often  mentioned  were  the  hazard 
from  leakage  of  a  high-pressure  gas  coolant  carrying  radioactive 
impurities,  the  difficulty  of  getting  large  blowers  quickly,  the 
large  amount  of  helium  required,  the  difficulty  of  loading  and 
unloading  uranium  from  the  pile,  and  the  relatively  low  power 
output  per  kilogram  of  uranium  metal.  These  considerations 
had  to  be  balanced  against  the  peculiar  disadvantages  of  a  water- 

114  Plutonium  Problem  February  1943 

cooled  plant,  principally  the  greater  complexity  of  the  pile  itself 
and  the  dangers  of  corrosion. 

7.17.  Like  so  many  decisions  in  this  project,  the  choice  between 
various  types  of  plant  had  to  be  based  on  incomplete  scientific 
information.  The  information  is  still  incomplete,  but  there  is 
general  agreement  that  water  cooling  was  the  wise  choice. 



7.18.  In   Chapter  II  of  this  report  we  attempted  to  define 
the  general  problem  of  the  uranium  project  as  it  appeared  in  the 
summer  of  1940.  We  now  wish  to  give  precise  definition  to  the 
problem  of  the  design  of  a  large-scale  plant  for  the  production 
of  plutonium.  The  objective  had  already  been  delimited  by 
decisions  as  to  scale  of  production,  type  of  plant,  and  site.  As  it 
then  stood,  the  specific  problem  was  to  design  a  water-cooled 
graphite-moderated  pile  (or  several  such  piles)  with  associated 
chemical  separation  plant  to  produce  a  specified,  relatively  large 
amount  of  plutonium  each  day,  the  plant  to  be  built  at  the  Han- 
ford  site  beside  the  Columbia  River.  Needless  to  say,  speed  of 
construction  and  efficiency  of  operation  were  prime  considerations. 


7.19.  The  lattices  we  have  been  describing  heretofore  con- 
sisted of  lumps  of  uranium  imbedded  in  the  graphite  moderator. 
There  are  two  objections  to  such  a  type  of  lattice  for  production 
purposes:  first,  it  is  difficult  to  remove  the  uranium  without  dis- 
assembling the  pile;  second,  it  is  difficult  to  concentrate  the 
coolant  at  the  uranium  lumps,  which  are  the  points  of  maximum 
production  of  heat.  It  was  fairly  obvious  that  both  these  diffi- 
culties could  be  avoided  if  a  rod  lattice  rather  than  a  point  lattice 
could  be  used,  that  is,  if  the  uranium  could  be  concentrated 
along  lines  passing  through  the  moderator  instead  of  being  situated 
merely  at  points.  There  was  little  doubt  that  the  rod  arrange- 
ment would   be  excellent  structurally  and   mechanically,   but 

Plutonium  Problem  February  1943  115 

there  was  real  doubt  as  to  whether  it  was  possible  to  build  such 
a  lattice  which  would  still  have  a  multiplication  factor  k  greater 
than  unity.  This  became  a  problem  for  both  the  theoretical  and 
experimental  physicists.  The  theoretical  physicists  had  to  compute 
what  was  the  optimum  spacing  and  diameter  of  uranium  rods; 
the  experimental  physicists  had  to  perform  exponential  experi- 
ments on  lattices  of  this  type  in  order  to  check  the  findings  of 
the  theoretical  group. 


7.20.  Once  the  idea  of  a  lattice  with  cylindrical  symmetry 
was  accepted,  it  became  evident  that  the  pile  could  be  unloaded 
and  reloaded  without  disassembly  since  the  uranium  could  be 
pushed  out  of  the  cylindrical  channels  in  the  graphite  moderator 
and  new  uranium  inserted.  The  decision  had  to  be  made  as  to 
whether  the  uranium  should  be  in  the  form  of  long  rods,  which 
had  advantages  from  the  nuclear-physics  point  of  view,  or  of 
relatively  short  cylindrical  pieces,  which  had  advantages  from 
the  point  of  view  of  handling.  In  either  case,  the  materials  would 
be  so  very  highly  radioactive  that  unloading  would  have  to  be 
carried  out  by  remote  control,  and  the  unloaded  uranium  would 
have  to  be  handled  by  remote  control  from  behind  shielding. 


7.21.  If  water  was  to  be  used  as  coolant,  it  would  have  to  be 
conveyed   to   the  regions   where   heat  was   generated   through 
channels  of  some  sort.  Since  graphite  pipes  were  not  practical, 
some  other  kind  of  pipe  would  have  to  be  used.  But  the  choice 
of  the  material  for  the  pipe,  like  the  choice  of  all  the  materials 
to  be  used  hi  the  pile,  was  limited  by  nuclear-physics  considera- 
tions. The  pipes  must  be  made  of  some  material  whose  absorption 
cross  section  for  neutrons  was  not  large  enough  to  bring  the  value 
of  k  below  unity.  Furthermore,  the  pipes  must  be  made  of  mate- 
rial which  would  not  disintegrate  under  the  heavy  density  of 
neutron  and  gamma  radiation  present  in  the  pile.  Finally,  the 
pipes   must  meet  all  ordinary  requirements   of  cooling-system 

116  Plutonium  Problem  February  1943 

pipes:  they  must  not  leak;  they  must  not  corrode;  they  must  not 

7.22.  From  the  nuclear-physics  point  of  view  there  were  seven 
possible  materials  (Pb,  Bi,  Be,  Al,  Mg,  Zn,  Sn),  none  of  which 
had  high  neutron-absorption  cross  sections.  No  beryllium  tubing 
was  available,  and  of  all  the  other  metals  only  aluminum  was 
thought  to  be  possible  from  a  corrosion  point  of  view.  But  it 
was  by  no  means  certain  that  aluminum  would  be  satisfactory, 
and  doubts  about  the  corrosion  of  the  aluminum  pipe  were  not 
settled  until  the  plant  had  actually  operated  for  some  time. 

7.23.  While  the  choice  of  material  for  the  piping  was  very 
difficult,  similar  choices — involving  both  nuclear-physics  criteria 
and  radiation-resistance  criteria — had  to  be  made  for  all  other 
materials  that  were  to  be  used  in  the  pile.  For  example,  the  elec- 
tric insulating  materials  to  be  used  in  any  instruments  buried 
in  the  pile  must  not  disintegrate  under  the  radiation.  In  certain 
instances  where  control  or  experimental  probes  had  to  be  in- 
serted and  removed  from  the  pile,  the  likelihood  had  to  be  borne 
in  mind  that  the  probes  would  become  intensely  radioactive  as 
a  result  of  their  exposure  in  the  pile  and  that  the  degree  to  which 
this  would  occur  would  depend  on  the  material  used. 

7.24.  Finally,  it  was  not  known  what  effect  the  radiation  fields 
in  the  pile  would  have  on  the  graphite  and  the  uranium.  It 
was  later  found  that  the  electric  resistance,  the  elasticity,  and  the 
heat  conductivity  of  the  graphite  all  change  with  exposure  to 
intense  neutron  radiation. 


7.25.  The  most  efficient  cooling  procedure  would  have  been 
to  have  the  water  flowing  in  direct  contact  with  the  uranium  in 
which  the  heat  was  being  produced.  Indications  were  that  this 
was  probably  out  of  the  question  because  the  uranium  would 
react  chemically  with  the  water,  at  least  to  a  sufficient  extent  to 
put  a  dangerous  amount  of  radioactive  material  into  solution 
and  probably  to  the  point  of  disintegrating  the  uranium  slugs. 
Therefore  it  was  necessary  to  find  some  method  of  protecting  the 

Plutonium  Problem  February  1943  117 

uranium  from  direct  contact  with  the  water.  Two  possibilities 
were  considered:  one  was  some  sort  of  coating,  either  by  electro- 
plating or  dipping;  the  other  was  sealing  the  uranium  slug  in  a 
protective  jacket  or  "can."  Strangely  enough,  this  "canning 
problem"  turned  out  to  be  one  of  the  most  difficult  problems 
encountered  in  such  piles. 


7.26.  The  problem  of  dissipating  thousands  of  kilowatts  of 
energy  is  by  no  means  a  small  one.  How  much  water  was  needed 
depended,  of  course,  on  the  maximum  temperature  to  which  the 
water  could  safely  be  heated  and  the  maximum  temperature  to 
be  expected  in  the  intake  from  the  Columbia  River;  certainly 
the  water  supply  requirement  was  comparable  to  that  of  a  fair- 
sized  city.  Pumping  stations,  filtration  and  treatment  plants  all 
had  to  be  provided.  Furthermore,  the  system  had  to  be  a  very 
reliable  one;  it  was  necessary  to  provide  fast-operating  controls 
to  shut  down  the  chain-reacting  unit  in  a  hurry  in  case  of  failure 
of  the  water  supply.  If  it  was  decided  to  use  "once-through" 
cooling  instead  of  recirculation,   a  retention  basin  would  be 
required  so  that  the  radioactivity  induced  in  the  water  might 
die  down  before  the  water  was  returned  to  the  river.  The  volume 
of  water  discharged  was  going  to  be  so  great  that  such  problems 
of  radioactivity  were  important,   and   therefore  the  minimum 
time  that  the  water  must  be  held  for  absolute  safety  had  to  be 


7.27.  The  control  problem  was  very  similar  to  that  discussed  in 
connection  with  the  first  chain-reacting  pile  except  that  every- 
thing was  on  a  larger  scale  and  was,  therefore,  potentially  more 
dangerous.  It  was  necessary  to  provide  operating  controls  which 
would  automatically  keep  the  pile  operating  at  a  determined 
power  level.  Such  controls  had  to  be  connected  with  instruments 
in  the  pile  which  would  measure  neutron  density  or  some  other 
property  which  indicated  the  power  level.  There  would  also  have 

118  Plutonium  Problem  February  1943 

to  be  emergency  controls  which  would  operate  almost  instan- 
taneously if  the  power  level  showed  signs  of  rapid  increase  or  if 
there  was  any  interruption  of  the  water  supply.  It  was  highly 
desirable  that  there  be  some  means  of  detecting  incipient  diffi- 
culties such  as  the  plugging  of  a  single  water  tube  or  a  break  in 
the  coating  of  one  of  the  uranium  slugs.  All  these  controls  and 
instruments  had  to  be  operated  from  behind  the  thick  shielding 
walls  described  below. 


7.28.  As  we  have  mentioned  a  number  of  times,  the  radiation 
given  off  from  a  pile  operating  at  a  high  power  level  is  so  strong 
as  to  make  it  quite  impossible  for  any  of  the  operating  personnel 
to  go  near  the  pile.  Furthermore,  this  radiation,  particularly  the 
neutrons,  has  a  pronounced  capacity  for  leaking  out  through 
holes  or  cracks  in  barriers.  The  whole  of  a  power  pile  therefore 
has  to  be  enclosed  in  very  thick  walls  of  concrete,  steel,  or  other 
absorbing  material.  But  at  the  same  time  it  has  to  be  possible  to 
load  and  unload  the  pile  through  these  shields  and  to  carry  the 
water  supply  in  and  out  through  the  shields.  The  shields  should 
not  only  be  radiation-tight  but  air-tight  since  air  exposed  to  the 
radiation  in  the  pile  would  become  radioactive. 

7.29.  The  radiation  dangers  that  require  shielding  in  the  pile 
continue  through  a  large  part  of  the  separation  plant.  Since  the 
fission  products  associated  with  the  production  of  the  plutonium 
are  highly  radioactive,  the  uranium  after  ejection  from  the  pile 
must  be  handled  by  remote  control  from  behind  shielding  and 
must  be  shielded  during  transportation  to  the  separation  plant. 
All  the  stages  of  the  separation  plant,  including  analyses,  must 
be  handled  by  remote  control  from  behind  shields  up  to  the 
point  where  the  plutonium  is  relatively  free  of  radioactive  fission 


7.30.  The  problem  of  maintenance  is  very  simply  stated.  There 
could  not  be  any  maintenance  inside  the  shield  or  pile  once  the 

Plutonium  Problem  February  1943  119 

pile  had  operated.  The  same  remark  applies  to  a  somewhat  lesser 
extent  to  the  separation  unit,  where  it  was  probable  that  a  shut- 
down for  servicing  could  be  effected,  provided,  of  course,  that 
adequate  remotely-controlled  decontamination  processes  were 
carried  out  in  order  to  reduce  the  radiation  intensity  below  the 
level  dangerous  to  personnel.  The  maintenance  problem  for  the 
auxiliary  parts  of  the  plant  was  normal  except  for  the  extreme 
importance  of  having  stand-by  pumping  and  power  equipment 
to  prevent  a  sudden  accidental  breakdown  of  the  cooling  system. 


7.31.  Evidently  the  amount  of  plutonium  in  an  undisturbed 
operating  pile  increases  with  time  of  operation.  Since  Pu-239 
itself  undergoes  fission  its  formation  tends  to  maintain  the  chain 
reaction,  while  the  gradual  disappearance  of  the  U-235  and  the 
appearance  of  fission  products  with  large  neutron  absorption 
cross  sections  tend  to  stop  the  reaction.  The  determination  of 
when  a  producing  pile  should  be  shut  down  and  the  plutonium 
extracted  involves  a  nice  balancing  of  these  factors  against  tune 
schedules,    material    costs,    separation-process    efficiency,    etc. 
Strictly  speaking,  this  problem  is  one  of  operation  rather  than 
of  design  of  the  plant,  but  some  thought  had  to  be  given  to  it  in 
order  to  plan  the  flow  of  uranium  slugs  to  the  pile  and  from  the 
pile  to  the  separation  plant. 


7.32.  We  have  been  speaking  of  the  production  capacity  of  the 
plant  only  in  terms  of  overall  production  rate.  Naturally,  a  given 
rate  of  production  might  be  achieved  in  a  single  large  pile  or  in 
a  number  of  smaller  ones.  The  principal  advantage  of  the  smaller 
piles  would  be  the  reduction  in  construction  time  for  the  first 
pile,  the  possibility  of  making  alterations  hi  later  piles,  and — 
perhaps    most    important — the    improbability    of  simultaneous 
breakdown  of  all  piles.  The  disadvantage  of  small  piles  is  that 
they    require    disproportionately    large    amounts    of   uranium, 

120  Plutonium  Problem  February  1943 

moderator,  etc.  There  is,  in  fact,  a  preferred  "natural  size"  of 
pile  which  can  be  roughly  determined  on  theoretical  grounds. 


7.33.  As  we  have  already  pointed  out,  the  slugs  coming  from 
the  pile  are  highly  radioactive  and  therefore  must  be  processed 
by  remote  control  in  shielded  compartments.  The  general  scheme 
to  be  followed  was  suggested  in  the  latter  part  of  1 942,  particularly 
in  connection  with  plans  for  the  Clinton  separation  plant.  This 
scheme  was  to  build  a  "canyon"  which  would  consist  of  a  series 
of  compartments  with  heavy  concrete  walls  arranged  in  a  line 
and  almost  completely  buried  in  the  ground.  Each  compartment 
would  contain  the  necessary  dissolving  or  precipitating  tanks  or 
centrifuges.  The  slugs  would  come  into  the  compartment  at  one 
end  of  the  canyon;  they  would  then  be  dissolved  and  go  through 
the  various  stages  of  solution,  precipitation,  oxidation,  or  reduc- 
tion, being  pumped  from  one  compartment  to  the  next  until  a 
solution  of  plutonium  free  from  uranium  and  fission  products 
came  out  in  the  last  compartment.  As  in  the  case  of  the  pile, 
everything  would  be  operated  by  remote  control  from  above 
ground,  but  the  operations  would  be  far  more  complicated  than 
in  the  case  of  the  pile.  However,  as  far  as  the  chemical  operations 
themselves  were  concerned,  their  general  nature  was  not  so  far 
removed   from   the   normal   fields   of  activity  of  the   chemists 


7.34.  In  the  first  stages  of  the  separation  process  even  the 
routine  analysis  of  samples  which  was  necessary  in  checking  the 
operation  of  the  various  chemical  processes  had  to  be  done  by 
remote  control.  Such  testing  was  facilitated,  however,  by  use  of 
radioactive  methods  of  analysis  as  well  as  conventional  chemical 


7.35.  The  raw  material  (uranium)  is  not  dangerously  radio- 
active. The  desired  product  (plutonium)  does  not  give  off  pene- 

Plutonium  Problem  February  1943  121 

trating  radiation,  but  the  combination  of  its  alpha-ray  activity 
and  chemical  properties  makes  it  one  of  the  most  dangerous  sub- 
stances known  if  it  once  gets  into  the  body.  However,  the  really 
troublesome  materials  are  the  fission  products,  i.e.,  the  major 
fragments  into  which  uranium  is  split  by  fission.  The  fission 
products  are  very  radioactive  and  include  some  thirty  elements. 
Among  them  are  radioactive  xenon  and  radioactive  iodine. 
These  are  released  in  considerable  quantity  when  the  slugs  are 
dissolved  and  must  be  disposed  of  with  special  care.  High  stacks 
must  be  built  which  will  carry  off  these  gases  along  with  the  acid 
fumes  from  the  first  dissolving  unit,  and  it  must  be  established 
that  the  mixing  of  the  radioactive  gases  with  the  atmosphere  will 
not  endanger  the  surrounding  territory.  (As  in  all  other  matters 
of  health,  the  tolerance  standards  that  were  set  and  met  were  so 
rigid  as  to  leave  not  the  slightest  probability  of  danger  to  the 
health  of  the  community  or  operating  personnel.) 

7.36.  Most  of  the  other  fission  products  can  be  retained  in 
solution  but  must  eventually  be  disposed  of.  Of  course,  possible 
pollution  of  the  adjacent  river  must  be  considered.  (In  fact,  the 
standards  of  safety  set  and  met  with  regard  to  river  pollution 
were  so  strict  that  neither  people  nor  fish  down  the  river  can 
possibly  be  affected.) 


7.37.  Evidently,  even  if  the  uranium  were  left  in  the  pile  until 
all  the  U-235  had  undergone  fission,  there  would  still  be  a  large 
amount  of  U-238  which  had  not  been  converted  to  plutonium. 
Actually  the  process  is  stopped  long  before  this  stage  is  reached. 
Uranium  is  an  expensive  material  and  the  total  available  supply 
is  seriously  limited.  Therefore  the  possibility  of  recovering  it  after 
the  plutonium  is  separated  must  be  considered.  Originally  there 
was  no  plan  for  recovery,  but  merely  the  intention  of  storing 
the  uranium  solution.  Later,  methods  of  large-scale  recovery 
were  developed 

122  Plutonium  Problem  February  1943 


7.38.  An  unusual  feature  of  the  chemical  processes  involved 
was  that  these  processes  occur  in  the  presence  of  a  high  density 
of  radiation.  Therefore  the  containers  used  may  corrode  more 
rapidly  than  they  would  under  normal  circumstances.  Further- 
more, any  such  corrosion  will  be  serious  because  of  the  difficulty 
of  access.  For  a  long  time,  information  was  sadly  lacking  on  these 


7.39.  The    chemical    reactions    proposed   for    an    extraction 
process  were,  of  course,  tested  in  the  laboratory.  However,  they 
could  not  be  tested  with  appreciable  amounts  of  plutonium  nor 
could  they  be  tested  in  the  presence  of  radiation  of  anything  like 
the  expected  intensity.  Therefore  it  was  realized  that  a  process 
found  to  be  successful  in  the  laboratory  might  not  work  in  the 


7.40.  The  description  given  above  as  to  what  was  to  happen 
in  the  successive  chambers  in  the  canyon  was  very  vague.  This 
was  necessarily  so,  since  even  by  January  1943  no  decision  had 
been  made  as  to  what  process  would  be  used  for  the  extraction 
and  purification  of  plutonium.  The  major  problem  before  the 
Chemistry  Division  of  the  Metallurgical  Laboratory  was  the 
selection  of  the  best  process  for  the  plant. 


7.41.  Besides  the  hazards  normally  present  during  construction 
and  operation  of  a  large  chemical  plant,  dangers  of  a  new  kind 
were  expected  here.  Two  types  of  radiation  hazard  were  antici- 
pated— neutrons   generated   in   the   pile,   and   alpha   particles, 
beta  particles,  and  gamma  rays  emitted  by  products  of  the  pile. 
Although  the  general  effects  of  these  radiations  had  been  proved 

Plutonium  Problem  February  1943  123 

to  be  similar  to  those  of  X-rays,  very  little  detailed  knowledge 
was  available.  Obviously  the  amounts  of  radioactive  material 
to  be  handled  were  many  times  greater  than  had  ever  been  en- 
countered before. 

7.42.  The  health  group  had  to  plan  three  programs:  (1)  pro- 
vision of  instruments  and  clinical  tests  to  detect  any  evidence  of 
dangerous  exposure  of  the  personnel;  (2)  research  on  the  effects 
of  radiation  on  persons,  instruments,  etc.;  and  (3)  estimates  of 
what  shielding  and  safety  measures  must  be  incorporated  in  the 
design  and  plan  of  operation  of  the  plant. 


7.43.  Although  we  were  embarking  on  a  major  enterprise  to 
produce  plutonium,  we  still  had  less  than  a  milligram  to  study 
and  still  had  only  limited  familiarity  with  its  properties.  The 
study  of  plutonium,  therefore,  remained  a  major  problem  of  the 
Metallurgical  Laboratory. 


7.44.  Evidently  the  operation  of  a  full-scale  plant  of  the  type 
planned  would  require  a  large  and  highly  skilled  group  of  oper- 
ators.  Although   du    Pont   had   a   tremendous    background   of 
experience  in  the  operation  of  various  kinds  of  chemical  plant, 
this  was  something  new  and  it  was  evident  that  operating  per- 
sonnel would  need  special  training.  Such  training  was  carried 
out  partly  in  Chicago  and  its  environs,  but  principally  at  the 
Clinton  Laboratories. 


7.45.  In  the  preceding  paragraphs  of  this  chapter  we  have 
outlined    the    problems    confronting    the    group    charged    with 
designing  and  building  a  plutonium  production  plant.  In  Chapter 
VI  the  progress  in  this  field  up  to  the  end  of  1942  was  reviewed. 
Throughout  these  chapters  it  is  made  clear  that  a  great  deal  more 

124  Plutonium  Problem  February  1943 

information  was  required  to  assure  the  success  of  the  plant. 
Such  answers  as  had  been  obtained  to  most  of  the  questions  were 
only  tentative.  Consequently  research  had  to  be  pushed  simul- 
taneously with  planning  and  construction. 


7.46.  To  meet  the  need  for  further  information,  research  pro- 
grams were  laid  out  for  the  Metallurgical  Laboratory  and  the 
Clinton  Laboratory.  The  following  passage  is  an  excerpt  from 
the  1943  program  of  the  Metallurgical  Project: 

"Product  Production  Studies.  These  include  all  aspects  of  the  re- 
search, development  and  semi-works  studies  necessary  for  the 
design,  construction,  and  operation  of  chain-reacting  piles  to 
produce  plutonium  or  other  materials. 

Pile  Characteristics.  Theoretical  studies  and  experiments  on 
lattice  structures  to  predict  behavior  in  high-level  piles,  such 
as  temperature  and  barometric  effects,  neutron  character- 
istics, pile  poisoning,  etc. 

Control  of  Reacting  Units.  Design  and  experimental  tests  of 
devices  for  controlling  rate  of  reaction  in  piles. 
Cooling  of  Reacting  Units.  Physical  studies  of  coolant  material, 
engineering  problems  of  circulation,  corrosion,  erosion,  etc. 
Instrumentation.  Development  of  instruments  and  technique 
for   monitoring   pile   and  surveying  radiation   throughout 
plant  area. 

Protection.  Shielding,  biological  effects  of  radiation  at  pile 
and  clinical  effects  of  operations  associated  with  pile. 
Materials.  Study  of  physical  (mechanical  and  nuclear)  prop- 
erties of  construction  and  process  materials  used  in  pile 
construction  and  operation. 

Activation  Investigations.  Production  of  experimental  amounts 
of  radioactive  materials  in  cyclotron  and  in  piles  and  study 
of  activation  of  materials  by  neutrons,  protons,  electrons, 
gamma  rays,  etc. 

Plutonium  Problem  February  1943  125 

Pile  Operation.  Study  of  pile  operation  procedures  such  as 
materials  handling,  instrument  operation,  etc. 
Process  Design.  Study  of  possible  production  processes  as  a 
whole  leading  to  detailed  work  in  other  categories. 

"Product  Recovery  Studies.  These  include  all  aspects  of  the  work 
necessary  for  the  development  of  processes  for  the  extraction  of 
plutonium  and  possible  by-products  from  the  pile  material  and 
their  preparation  in  purified  form.  Major  effort  at  the  Metal- 
lurgical Laboratory  will  be  on  a  single  process  to  be  selected  by 
June  1,  1943  for  the  production  of  plutonium,  but  alternatives 
will  continue  to  be  studied  both  at  the  Metallurgical  Laboratory 
and  Clinton  with  whatever  manpower  is  available. 

Separation.  Processes  for  solution  of  uranium,  extraction  of 
plutonium  and  decontamination  by  removal  of  fission 

Concentration,  Purification  and  Product  Reduction.  Processes  lead- 
ing to  production  of  plutonium  as  pure  metal,  and  study  of 
properties  of  plutonium  necessary  to  its  production. 
Wastes.  Disposal  and  possible  methods  of  recovery  of  fission 
products  and  metal  from  wastes. 

Instrumentation.  Development  and  testing  of  instruments  for 
monitoring  chemical  processes  and  surveying  radiation 
throughout  the  area. 

Protection.  Shi-elding  studies,  determination  of  biological 
effects  of  radioactive  dusts,  liquids,  solids,  and  other  process 
materials,  and  protective  measures. 

Materials.  Corrosion  of  equipment  materials,  and  radiation 
stability.  Necessary  purity  and  purity  analysis  of  process 
materials,  etc. 

Recovery  of  Activated  Materials.  Development  of  methods  and 
actual  recovery  of  activated  material  (tracers,  etc.)  from 
cyclotron  and  pile-activated  materials. 

126  Plutonium  Problem  February  1943 

Operations  Studies.  Equipment  performance,  process  control, 
material  handling  operations,  etc. 

Process  Design.  Study  of  product  recovery  processes  as  a 
whole  (wet  processes,  physical  methods)  leading  to  detailed 
work  in  other  categories. 

"Fundamental  Research.  Studies  of  the  fundamental  physical, 
chemical  and  biological  phenomena  occurring  in  chain-reacting 
piles,  and  basic  properties  of  all  materials  involved.  Although 
the  primary  emphasis  at  Clinton  is  on  the  semi-works  level, 
much  fundamental  research  will  require  Clinton  conditions  (high 
radiation  intensity,  large  scale  processes). 

Nuclear  Physics.  Fundamental  properties  of  nuclear  fission 
such  as  cross  section,  neutron  yield,  fission  species,  etc. 
Other  nuclear  properties  important  to  processes,  such  as 
cross  sections,  properties  of  moderators,  neutron  effect  on 
materials,  etc. 

General    Physics.    Basic    instrument    (electronic,    ionization, 
optical,  etc.)  research,  atomic  mass  determinations,  neutron, 
a,  /3,  7  radiation  studies,  X-ray  investigations,  etc. 
Radiation  Chemistry.  Effects  of  radiation  on  chemical  proc- 
esses and  chemical  reactions  produced  by  radiation. 
Nuclear  Chemistry.  Tracing  of  fission  products,  disintegration 
constants,  chains,  investigation  of  nuclei  of  possible  use  to 

Product  Chemistry.  Chemical  properties  of  various  products 
and  basic  studies  in  separation  and  purification  of  products. 
General  Chemistry.  Chemistry  of  primary  materials  and  mate- 
rials associated  with  process,  including  by-products. 
General  Biology.  Fundamental  studies  of  effects  of  radiation 
on  living  matter,  metabolism  of  important  materials,  etc. 
Clinical   Investigations.    Basic    investigations,  such  as  hema- 
tology,  pathology,  etc. 
Metallurgical  Studies.  Properties  of  U,  Pu,  Be,  etc. 

Plutonium  Problem  February  1943  127 

Engineering  Studies.  Phenomena  basic  to  corrosion  and  similar 
studies  essential  to  continued  engineering  development  of 

7.47.  An  examination  of  this  program  gives  an  idea  of  the 
great  range  of  investigations  which  were  considered  likely  to 
give  relevant  information.   Many  of  the  topics  listed  are  not 
specific  research  problems  such  as  might  be  solved  by  a  small 
team  of  scientists  working  for  a  few  months  but  are  whole  fields 
of  investigation  that  might  be  studied  with  profit  for  years.  It  was 
necessary  to  pick  the  specific  problems  that  were  likely  to  give 
the  most  immediately  useful  results  but  at  the  same  time  it  was 
desirable  to  try  to  uncover  general  principles.  For  example,  the 
effect  of  radiation  on   the   properties   of  materials  ("radiation 
stability")  was  almost  entirely  unknown.  It  was  necessary  both 
to  make  empirical  tests  on  particular  materials  that  might  be 
used  in  a  pile  and  to  devise  general  theories  of  the  observed 
effects.  Every  effort  was  made  to  relate  all  work  to  the  general 
objective:  a  successful  production  plant. 


7.48.  There  have  been  many  changes  in  the  organization  and 
personnel  of  the  project.  During  most  of  the  period  of  construc- 
tion at  Clinton  and  Hanford,  A.  H.  Compton  was  director  of  the 
Metallurgical  Project;  S.  K.  Allison  was  director  of  the  Metal- 
lurgical Laboratory  at  Chicago;  and  M.  D.  Whitaker  was  direc- 
tor of  the  Clinton  Laboratory.  The  Chicago  group  was  organized 
in  four  divisions:   physics,  chemistry,   technology,  and  health. 
Later  the  Physics  Division  was  split  into  general  physics  and 
nuclear  physics.  R.  L.  Doan  was  research  director  at  Clinton 
but  there  was  no  corresponding  position  at  Chicago.  Among 
others  who  have  been  associate  or  assistant  laboratory  or  project 
directors  or  have  been  division  directors  are  S.  T.  Cantril,  C.  M. 
Cooper,    F.    Daniels,    A.   J.    Dempster,   E.   Fermi,  J.   Franck, 
N.  Hilberry,  T.  R.  Hogness,  W.  C.  Johnson,  H.  D.  Smyth,  J.  C. 
Stearns,  R.  S.  Stone,  H.  C.  Vernon,  W.  W.  Watson,  and  E. 

128  Plutonium  Problem  February  1943 

Wigner.  Beginning  in  1943  C.  H.  Thomas  of  the  Monsanto 
Chemical  Company  acted  as  chairman  of  a  committee  on  the 
Chemistry  and  Metallurgy  of  Plutonium.  This  committee  cor- 
related the  activities  of  the  Metallurgical  Laboratory  with  those 
at  Los  Alamos  (see  Chapter  XII)  and  elsewhere.  Later  the 
Monsanto  Chemical  Company  did  some  work  on  important 
special  problems  arising  in  connection  with  the  Los  Alamos  work. 

7.49.  It  was  the  responsibility  of  these  men  to  see  that  the 
research  program  described  above  was  carried  out  and  that 
significant  results  were  reported  to  du  Pont.  It  was  their  responsi- 
bility also  to  answer  questions  raised  by  du  Pont  and  to  approve 
or  criticize  plans  submitted  by  du  Pont. 


7.50.  Since  du  Pont  was  the  design  and  construction  organiza- 
tion and  the  Metallurgical  Laboratory  was  the  research  organiza- 
tion, it  was  obvious  that  close  cooperation  was  essential.  Not  only 
did  du  Pont  need  answers  to  specific  questions,  but  they  could 
benefit  by  criticism  and  suggestions  on  the  many  points  where 
the  Metallurgical  group  was  especially  well-informed.  Similarly, 
the  Metallurgical  group  could  profit  by  the  knowledge  of  du  Pont 
on  many  technical  questions  of  design,  construction,  and  opera- 
tion. To  promote  this  kind  of  cooperation  du  Pont  stationed  one 
of  their  physicists,  J.  B.  Miles,  at  Chicago,  and  had  many  other 
du  Pont  men,  particularly  C.  H.  Greenewalt,  spend  much  of 
their  time  at  Chicago.  Miles  and  Greenewalt  regularly  attended 
meetings   of  the   Laboratory   Council.   There   was   no   similar 
reciprocal  arrangement  although  many  members  of  the  labora- 
tory visited  Wilmington  informally.  In  addition,  J.  A.  Wheeler 
was  transferred  from  Chicago  to  Wilmington  and   became  a 
member  .of  the  du  Pont  staff.  There  was,  of  course,  constant 
exchange  of  reports  and  letters,  and  conferences  were  held  fre- 
quently between  Compton  and  R.  Williams  of  du  Pont.  Whitaker 
spent  much  of  his  time  at  Wilmington  during  the  period  when  the 
Clinton  plant  was  being  designed  and  constructed. 

Plutonium  Problem  February  1943  129 


7.51.  By  January  1943,  the  decision  had  been  made  to  build 
a  plutonium  production  plant  with  a  large  capacity.  This  meant 
a  pile  developing  thousands  of  kilowatts  and  a  chemical  separa- 
tion plant  to  extract  the  product.  The  du  Pont  Company  was  to 
design,  construct,  and  operate  the  plant;  the  Metallurgical  Labor- 
atory was  to  do  the  necessary  research.  A  site  was  chosen  on  the 
Columbia  River  at  Hanford,  Washington.  A  tentative  decision 
to  build  a  helium-cooled  plant  was  reversed  in  favor  of  water- 
cooling.  The  principal  problems  were  those  involving  lattice 
design,  loading  and  unloading,  choice  of  materials  particularly 
with  reference  to  corrosion  and  radiation,  water  supply,  controls 
and  instrumentation,  health  hazards,  chemical  separation  process, 
and  design  of  the  separation  plant.  Plans  were  made  for  the  neces- 
sary fundamental  and  technical  research  and  for  the  training  of 
operators.  Arrangements  were  made  for  liaison  between  du  Pont 
and  the  Metallurgical  Laboratory. 

JANUARY  1943  TO  JUNE  1945 


8.1.  The  necessity  for  pushing  the  design  and  construction  of 
the  full-scale  plutonium  plant  simultaneously  with  research  and 
development  inevitably  led  to  a  certain  amount  of  confusion 
and  inefficiency.  It  became  essential  to  investigate  many  alter- 
native processes.  It  became  necessary  to  investigate  all  possible 
causes  of  failure  even  when  the  probability  of  their  becoming 
serious  was  very  small.  Now  that  the  Hanford  plant  is  producing 
plutonium  successfully,  we  believe  it  is  fair  to  say  that  a  large 
percentage  of  the  results  of  investigation  made  between  the  end 
of  1942  and  the  end  of  1944  will  never  be  used — at  least  not  for 
the  originally  intended  purposes.  Nevertheless  had  the  Hanford 
plant  run  into  difficulties,  any  one  of  the  now  superfluous  in- 
vestigations might  have  furnished  just  the  information  required 
to  convert  failure  into  success.  Even  now  it  is  impossible  to  say 
that  future  improvements  may  not  depend  on  the  results  of 
researches  that  seem  unimportant  today. 

8.2.  It  is  estimated  that  thirty  volumes  will  be  required  for  a 
complete  report  of  the  significant  scientific  results  of  researches 
conducted   under    the    auspices    of  the    Metallurgical    Project. 
Work  was  done  on  every  item  mentioned  on  the  research  program 
presented  in  the  last  chapter.  In  the  present  account  it  would  be 
obviously  impossible  to  give  more  than  a  brief  abstract  of  all 
these  researches.  We  believe  this  would  be  unsatisfactory  and 
that  it  is  preferable  to  give  a  general  discussion  of  the  chain- 
reacting  units  and  separation  plants  as  they  now  operate,  with 
some  discussion  of  the  earlier  developments. 


8.3.  In  Chapter  I  and  other  early  chapters  we  have  given 
brief  accounts  of  the  fission  process,  pile  operation,  and  chemical 


Plutonium  Problem  to  June  1945  131 

separation.  We  shall  now  review  these  topics  from  a  somewhat 
different  point  of  view  before  describing  the  plutonium  produc- 
tion plants  themselves. 

8.4.  The  operation  of  a  pile  depends  on  the  passage  of  neutrons 
through  matter  and  on  the  nature  of  the  collisions  of  neutrons 
with  the  nuclei  encountered.  The  collisions  of  principal  impor- 
tance are  the  following: 

I.  Collisions  in  which  neutrons  are  scattered  and  lose  appreci- 
able amounts  of  energy,  (a)  Inelastic  collisions  of  fast  neutrons 
with  uranium  nuclei,  (b)  Elastic  collisions  of  fast  or  moderately 
fast  neutrons  with  the  light  nuclei  of  the  moderator  material; 
these  collisions  serve  to  reduce  the  neutron  energy  to  very  low 
(so-called  thermal)  energies. 

II.  Collisions  in  which  the  neutrons  are  absorbed,  (a)  Colli- 
sions which  result  in  fission  of  nuclei  and  give  fission  products  and 
additional  neutrons,  (b)  Collisions  which  result  in  the  formation 
of  new   nuclei   which   subsequently   disintegrate   radioactively 
(e.g.,  wU239  which  produces  94Pu239). 

8.5.  Only  the  second  class  of  collision  requires  further  dis- 
cussion. As  regards  collisions  of  Type  II  (a),  the  most  important 
in  a  pile  are  the  collisions  between  neutrons  and  U-235,  but  the 
high-energy  fission  of  U-238  and  the  thermal  fission  of  Pu-239 
also  take  place.  Collisions  of  Type  II  (b)  are  chiefly  those  between 
neutrons  and  U-238.  Such  collisions  occur  for  neutrons  of  all 
energies,  but  they  are  most  likely  to  occur  for  neutrons  whose 
energies  lie  in  the  "resonance"  region  located  somewhat  above 
thermal  energies.  The  sequence  of  results  of  the  Type  II   (b) 
collision  is  represented  as  follows: 

MU238  +  on1  ->  92U239  +  gamma  rays 
92U239  ^j— »  93Np239  -f  _ie° 

93NP239TTdl^ 94Fu239  +  -ie°  +  gamma  rays 

8.6.  Any  other  non-fission  absorption  processes  are  important 
chiefly  because  they  waste  neutrons;  they  occur  in  the  moderator, 

132  Plutonium  Problem  to  June  1945 

in  U-235,  in  the  coolant,  in  the  impurities  originally  present,  in 
the  fission  products,  and  even  in  plutonium  itself. 

8.7.  Since   the  object  of  the  chain  reaction  is   to  generate 
plutonium,  we  would  like  to  absorb  all  excess  neutrons  in  U-238, 
leaving  just  enough  neutrons  to  produce  fission  and  thus   to 
maintain    the    chain   reaction.    Actually    the    tendency   of   the 
neutrons  to  be  absorbed  by  the  dominant  isotope  U-238  is  so 
great  compared  to  their  tendency  to  produce  fission  in  the  140- 
times-rarer  U-235  that  the  principal  design  effort  had  to  be 
directed  toward  favoring  the  fission  (as  by  using  a  moderator, 
a  suitable  lattice,  materials  of  high  purity,  etc.,)  in  order  to 
maintain  the  chain  reaction. 


8.8.  All  the  chain-reacting  piles  designed  by  the  Metallurgical 
Laboratory  or  with  its  cooperation  consist  of  four  categories  of 
material — the  uranium  metal,  the  moderator,  the  coolant,  and 
the  auxiliary  materials  such  as  water  tubes,  casings  of  uranium, 
control  strips  or  rods,  impurities,  etc.  All  the  piles  depend  on 
stray  neutrons  from  spontaneous  fission  or  cosmic  rays  to  initiate 
the  reaction. 

8.9.  Suppose  that  the  pile  were  to  be  started  by  simultaneous 
release    (in   the   uranium   metal)    of  N   high-energy   neutrons. 
Most  of  these  neutrons  originally  have  energies  above  the  thresh- 
old energy  of  fission  of  U-238.  However,  as  the  neutrons  pass 
back  and  forth  in  the  metal  and  moderator,  they  suffer  numerous 
inelastic  collisions  with  the  uranium  and  numerous  elastic  colli- 
sions with  the  moderator,  and  all  these  collisions  serve  to  reduce 
the  energies   below   that  threshold.   Specifically,   in   a   typical 
graphite-moderated  pile  a  neutron  that  has  escaped  from  the 
uranium  into  the  graphite  travels  on  the  average  about  2.5  cm 
between  collisions  and  makes  on  the  average  about  200  elastic 
collisions  before  passing  from  the  graphite  back  into  the  uranium. 
Since  at  each  such  collision  a  neutron  loses  on  the  average  about 
one  sixth  of  its  energy,  a  one-Mev  neutron  is  reduced  to  thermal 

*  See  drawing  facing  p.  35. 

Plutonium  Problem  to  June  1945  133 

energy  (usually  taken  to  be  0.025  electron  volt)  considerably 
before  completing  a  single  transit  through  the  graphite.  There 
are,  of  course,  many  neutrons  that  depart  from  this  average 
behavior,  and  there  will  be  enough  fissions  produced  by  fast 
neutrons  to  enhance  slightly  the  number  of  neutrons  present. 
The  enhancement  may  be  taken  into  account  by  multiplying 
the  original  number  of  neutrons  N  by  a  factor  €  which  is  called 
the  fast-fission  effect  or  the  fast-multiplication  factor. 

8.10.  As  the  average  energy  of  the  N€  neutrons  present  con- 
tinues to  fall,  inelastic  collision  in  the  uranium  becomes  unimpor- 
tant, the  energy  being  reduced  essentially  only  in  the  moderator. 
However,  the  chance  of  non-fission  absorption  (resonance  cap- 
ture) in  U-238  becomes  significant  as  the  intermediate  or  reson- 
ance energy  region  is  reached.   Actually  quite  a  number  of 
neutrons  in  this  energy  region  will  be  absorbed  regardless  of  choice 
of  lattice  design.  The  effect  of  such  capture  may  be  expressed  by 
multiplying  N«  by  a  factor  p,  (which  is  always  less  than  one) 
called  the  "resonance  escape  probability"  which  is  the  proba- 
bility that  a  given  neutron  starting  with  energy  above  the  reson- 
ance region  will  reach  thermal  energies  without  absorption  in 
U-238.  Thus  from  the  original  N  high-energy  neutrons  we  obtain 
Nep  neutrons  of  thermal  energy. 

8.11.  Once  a  neutron  has  reached  thermal  energy  the  chance 
that  it  will  lose  more  energy  by  collision  is  no  greater  than  the 
chance  that  it  will  gain  energy.  Consequently  the  neutrons  will 
remain  at  this  average  energy  until  they  are  absorbed.  In  the 
thermal-energy  region  the  chance  for  absorption  of  the  neutron 
by  the  moderator,  the  coolant  and  the  auxiliary  materials  is 
greater  than  at  higher  energies.  At  any  rate  it  is  found  that  we 
introduce  little  error  into  our  calculations  by  assuming  all  such 
unwanted  absorption  takes  place  in  this  energy  region.  We  now 
introduce  a  factor  f,  called  the  thermal  utilization  factor,  which 
is  defined  as  the  probability  that  a  given  thermal  neutron  will  be 
absorbed  in  the  uranium.  Thus  from  the  original  N  fast  neutrons 
we  have  obtained  Ntpf  thermal  neutrons  which  are  absorbed  by 

134  Plutonium  Problem  to  June  1945 

8.12.  Although  there  are  several  ways  in  which  the  normal 
mixture  of  uranium  isotopes  can  absorb  neutrons,  the  reader  may 
recall  that  we  defined  in  a  previous  chapter  a  quantity  77,  which 
is  the  number  of  fission  neutrons  produced  for  each  thermal 
neutron  absorbed  in  uranium  regardless  of  the  details  of  the 
process.  If,  therefore,  we  multiply  the  number  of  thermal  neu- 
trons absorbed  in  uranium,  Ncpf,  by  77,  we  have  the  number  of 
new  high  speed  neutrons  generated  by  the  original  N  high  speed 
neutrons  in  the  course  of  their  lives.  If  Nepfr;  is  greater  than  N, 
we  have  a  chain  reaction  and  the  number  of  neutrons  is  con- 
tinually increasing.  Evidently  the  product  epf?;  =  k^,  the  multi- 
plication factor  already  defined  in  Chapter  IV. 

8.13.  Note    that   no    mention    has    been    made    of  neutrons 
escaping  from   the  pile.   Such  mention   has   been   deliberately 
avoided  since  the  value  of  k^  as  defined  above  applies  to  an 
infinite  lattice.  From  the  known  values  of  k^  and  the  fact  that 
these  piles  do  operate,  one  finds  that  the  percentage  of  neutrons 
escaping  cannot  be  very  great.  As  we  saw  in  Chapter  II,  the 
escape  of  neutrons  becomes  relatively  less  important  as  the  size 
of  the  pile  increases.  If  it  is  necessary  to  introduce  in  the  pile  a 
large  amount  of  auxiliary  material  such  as  cooling-system  pipes, 
it  is  necessary  to  build  a  somewhat  larger  pile  to  counteract  the 
increase  in  absorption. 

8.14.  To  sum  up,  a  pile  operates  by  reducing  high-energy 
neutrons  to  thermal  energies  by  the  use  of  a  moderator-lattice 
arrangement,  then  allowing  the  thermal-energy  neutrons  to  be 
absorbed  by  uranium,  causing  fission  which  regenerates  further 
high-energy  neutrons.   The  regeneration  of  neutrons   is   aided 
slightly  by  the  fast  neutron  effect;  it  is  impeded  by  resonance 
absorption  during  the  process  of  energy  reduction,  by  absorption 
in  graphite  and  other  materials,  and  by  neutron  escape. 


8.15.  Even  at  the  high  power  level  used  in  the  Hanford  piles, 
only  a  few  grams  of  U-238  and  of  U-235  are  used  up  per  day  per 

Plutonium  Problem  to  June  1945  135 

million  grams  of  uranium  present.  Nevertheless  the  effects  of 
these  changes  are  very  important.  As  the  U-235  is  becoming 
depleted,  the  concentration  of  plutonium  is  increasing.  For- 
tunately, plutonium  itself  is  fissionable  by  thermal  neutrons  and 
so  tends  to  counterbalance  the  decrease  of  U-235  as  far  as  main- 
taining the  chain  reaction  is  concerned.  However,  other  fission 
products  are  being  produced  also.  These  consist  typically  of 
unstable  and  relatively  unfamiliar  nuclei  so  that  it  was  originally 
impossible  to  predict  how  great  an  undesirable  effect  they  would 
have  on  the  multiplication  constant.  Such  deleterious  effects  are 
called  poisoning. 


8.16.  There  are  two  main  parts  of  the  plutonium  production 
process  at  Hanford :  actual  production  in  the  pile,  and  separation 
of  the  plutonium  from  the  uranium  slugs  in  which  it  is  formed. 
We  turn  now  to  a  discussion  of  the  second  part,  the  separation 

8.17.  The  uranium  slugs  containing  plutonium  also  contain 
other  elements  resulting  from  the  fission  of  U-235.  When  a  U-235 
nucleus  undergoes  fission,  it  emits  one  or  more  neutrons  and 
splits  into  two  fragments  of  comparable  size  and  of  total  mass 
235  or  less.  Apparently  fission  into  precisely  equal  masses  rarely 
occurs,  the  most  abundant  fragments  being  a  fragment  of  mass 
number  between  134  and  144  and  a  fragment  of  mass  number 
between  100  and  90.  Thus  there  are  two  groups  of  fission  prod- 
ucts:  a  heavy  group  with  mass  numbers  extending  approxi- 
mately from  127  to  154,  and  a  light  group  from  approximately 
1 15  to  83.  These  fission  products  are  in  the  main  unstable  isotopes 
of  the  thirty  or  so  known  elements  in  these  general  ranges  of  mass 
number.    Typically   they   decay   by   successive   beta   emissions 
accompanied  by  gamma  radiation  finally  to  form  known  stable 
nuclei.  The  half-lives  of  the  various  intermediate  nuclei  range 
from  fractions  of  a  second  to  a  year  or  more;  several  of  the 
important  species  have  half-lives  of  the  order  of  a  month  or  so. 

136  Plutonium  Problem  to  June  1945 

About  twenty  different  elements  are  present  in  significant  con- 
centration. The  most  abundant  of  these  comprises  slightly  less 
than  10  per  cent  of  the  aggregate. 

8.18.  In  addition  to  radioactive  fission  products,  U-239  and 
Np-239  (intermediate  products  in  the  formation  of  plutonium) 
are  present  in  the  pile  and  are  radioactive.  The  concentrations 
of  all  these  products  begin  to  build  up  at  the  moment  the  pile 
starts  operating.  Eventually  the  rate  of  radioactive  decay  equals 
the  rate  of  formation  so  that  the  concentrations  become  constant. 
For  example,  the  number  of  atoms  of  U-239  produced  per  second 
is  constant  for  a  pile  operating  at  a  fixed  power  level.  According 
to  the  laws  of  radioactive  disintegration,  the  number  of  U-239 
atoms  disappearing  per  second  is  proportional  to  the  number  of 
such  atoms  present  and  is  thus  increasing  during  the  first  few 
minutes  or  hours  after  the  pile  is  put  into  operation.  Consequently 
there  soon  will  be  practically  as  many  nuclei  disintegrating  each 
second  as  are  formed  each  second.  Equilibrium  concentrations 
for  other  nuclei  will  be  approached  in  similar  manner,  the 
equilibrium  concentration  being  proportional  to  the  rate  of 
formation  of  the  nucleus  and  to  its  half-life.  Products  which  are 
stable  or  of  extremely  long  half-life  (e.g.,  plutonium)  will  steadily 
increase  in  concentration  for  a  considerable  time.  When  the  pile 
is  stopped,  the  radioactivity  of  course  continues,  but  at  a  con- 
tinually diminishing  absolute  rate.  Isotopes  of  very  short  half- 
life  may  "drop  out  of  sight"  in  a  few  minutes  or  hours;  others  of 
longer  half-life  keep  appreciably  active  for  days  or  months. 
Thus  at  any  time  the  concentrations  of  the  various  products  in  a 
recently  stopped  pile  depend  on  what  the  power  level  was,  on 
how  long  the  pile  ran,  and  on  how  long  it  has  been  shut  down. 
Of  course,  the  longer  the  pile  has  run,  the  larger  is  the  concentra- 
tion of  plutonium  and  (unfortunately)  the  larger  is  the  concentra- 
tion of  long-lived  fission  products.  The  longer  the  "cooling" 
period,  i.e.,  the  period  between  removal  of  material  from  the 
pile  and  chemical  treatment,  the  lower  is  the  radiation  intensity 
from  the  fission  products.  A  compromise  must  be  made  between 
such  considerations  as  the  desire  for  a  long  running  and  cooling 

Plutonium  Problem  to  June  1945  137 

time  on  the  one  hand  and  the  desire  for  early  extraction  of  the 
plutonium  on  the  other  hand. 

8.19.  Tables  can  be  prepared  showing  the  chemical  concentra- 
tions of  plutonium  and  the  various  fission  products  as  functions 
of  power  level,  length  of  operation,  and  length  of  cooling  period. 
The  half-life  of  the  U-239  is  so  short  that  its  concentration  be- 
comes negligible  soon  after  the  pile  shuts  down.  The  neptunium 
becomes  converted  fairly  rapidly  to  plutonium.  Of  course,  the 
total  weight  of  fission  products,  stable  and  unstable,  remains 
practically  constant  after  the  pile  is  stopped.  For  the  Clinton  and 
Hanford   operating  conditions   the  maximum   plutonium   con- 
centration attained  is  so  small  as  to  add  materially  to  the  diffi- 
culty of  chemical  separation. 


8.20.  The  problem  then  is  to  make  a  chemical  separation  at 
the  daily  rate  of,  say,  several  grams  of  plutonium  from  several 
thousand  grams  of  uranium  contaminated  with  large  amounts  of 
dangerously    radioactive    fission    products    comprising    twenty 
different  elements.   The  problem  is  especially  difficult  as  the 
plutonium  purity  requirements  are  very  high  indeed. 

8.21.  Four   types   of  method   for  chemical   separation   were 
examined:   volatility,   absorption,  solvent  extraction,   and  pre- 
cipitation.   The    work    on    absorption    and    solvent    extraction 
methods  has  been  extensive  and  such  methods  may  be  increas- 
ingly used  in  the  main  process  or  in  waste  recovery,  but  the  Han- 
ford  Plant  was  designed  for  a  precipitation  process. 

8.22.  *  The  phenomena  of  co-precipitation,  i.e.,  the  precipita- 
tion of  small  concentrations  of  one  element  along  with  a  "carrier" 
precipitate  of  some  other  element,  had  been  commonly  used  in 
radioactive  chemistry,  and  was  adopted  for  plutonium  separation. 
The  early  work  on  plutonium  chemistry,  confined  as  it  was  to 
minute  amounts  of  the  element,  made  great  use  of  precipitation 

*  Paragraphs  8.22-8.26  are  quoted  or  paraphrased  from  a  general  report 
of  the  Metallurgical  Laboratory  prepared  in  the  spring  of  1945. 

138  Plutonium  Problem  to  June  1945 

reactions  from  which  solubility  properties  could  be  deduced. 
It  was  therefore  natural  that  precipitation  methods  of  separation 
were  the  most  advanced  at  the  time  when  the  plant  design  was 
started.  It  was  felt  that,  should  the  several  steps  in  the  separations 
process  have  to  be  developed  partly  by  the  empirical  approach, 
there  would  be  less  risk  in  the  scale-up  of  a  precipitation  process 
than,  for  example,  of  one  involving  solid-phase  reactions.  In 
addition,  the  precipitation  processes  then  in  mind  could  be 
broken  into  a  sequence  of  repeated  operations  (called  cycles), 
thereby  limiting  the  number  of  different  equipment  pieces 
requiring  design  and  allowing  considerable  process  change 
without  equipment  change.  Thus,  while  the  basic  plant  design 
was  made  with  one  method  in  mind,  the  final  choice  of  a  different 
method  led  to  no  embarrassments. 

8.23.  Most  of  the  precipitation  processes  which  have  received 
serious  consideration  made  use  of  an  alternation  between  the 
(IV)   and   (VI)  oxidation  states  of  plutonium.   Such  processes 
involve  a  precipitation  of  plutonium  (IV)  with  a  certain  com- 
pound as  a  carrier,  then  dissolution  of  the  precipitate,  oxidation 
of  the  plutonium  to  the  (VI)  state,  and  reprecipitation  of  the 
carrier  compound  while  the  plutonium  (VI)  remains  in  solution. 
Fission  products  which  are  not  carried  by  these  compounds 
remain  in  solution  when  plutonium  (IV)  is  precipitated.  The 
fission  products  which  carry  are  removed  from  the  plutonium 
when   it  is   in   the    (VI)   state.    Successive   oxidation-reduction 
cycles    are   carried   out   until   the   desired   decontamination   is 
achieved.  The  process  of  elimination  of  the  fission  products  is 
called  decontamination  and  the  degree  of  elimination  is  tested 
by  measuring  the  change  in  radioactivity  of  the  material. 


8.24.  It  is  possible  to  combine  or  couple  the  various  types  of 
process.  Some  advantages  may  be  gained  in  this  way  since  one 
type  of  process  may  supplement  another.  For  example,  a  process 
which  gives  good  decontamination  might  be  combined  advan- 

MAN  MA  n  AN   UiS'l  KiCl 


"rf^fc^l**  -% 

*#*  _ 

Above:  Administration  Building  for  the  Manhattan  Engineer  District  at  Oak  Ridge. 
Tenn.  Below:  Hanford,  near  Pasco,  Wash.,  which  at  one  time  housed  thousands  of 
workers  who  built  the  plants  at  the  Hanford  Engineer  Works.  Now  a  ghost  town. 

Site  diagram  of  the  Hanford  Engineer  Works  near  Pasco,  Wash. 


-r  V 






Initial  test  of  the  atomic  bomb  in  New  Mexico  on  July  16,  from  a  distance  of  6  miles. 
Above:  The  start  of  the  explosion.  This  small  cloud  later  rose  to  a  height  of  40,000  feet. 
Center:  Multi-colored  cloud  from  the  explosion.  Black  areas  were  brighter  than  the 
sun  itself,  according  to  observers.  Below.  A  later  stage  of  the  development  of  the  cloud. 

Plutonium  Problem  to  June  1945  139 

tageously  with  one  which,  while  inefficient  for  decontamination, 
would  be  very  efficient  for  separation  from  uranium. 

8.25.  At  the  time  when  it  became  necessary  to  decide  on  the 
process  to  serve  as  the  basis  for  the  design  of  the  Hanford  plant 
(June  1943),  the  choice,  for  reasons  given  above,  was  limited  to 
precipitation  processes  and  clearly  lay  between  two  such  processes. 
However,  the  process  as  finally  chosen  actually  represented  a 
combination  of  the  two. 

8.26.  The  success  of  the  separation  process  at  Hanford  has 
exceeded  all  expectations.  The  high  yields  and  decontamination 
factors  and  the  relative  ease  of  operation  have  amply  demon- 
strated the  wisdom  of  its  choice  as  a  process.  This  choice  was 
based  on  a  knowledge  of  plutonium  chemistry  which  had  been 
gleaned  from  less  than  a  milligram  of  plutonium.  Further  devel- 
opments may  make  the  present  Hanford  process  obsolete,  but 
the  principal  goal,  which  was  to  have  a  workable  and  efficient 
process  for  use  as  soon  as  the  Hanford  piles  were  delivering  pluton- 
ium, has  been  attained. 


8.27.  The  Argonne  Laboratory  was  constructed  early  in  1943 
outside  Chicago.  The  site,  originally  intended  for  a  pilot  plant, 
was  later  considered  to  be  too  near  the  city  and  was  used  for 
reconstructing  the  so-called  West  Stands  pile  which  was  orig- 
inally built  on  the  University  of  Chicago  grounds  and  which  was 
certainly  innocuous.  Under  the  direction  of  E.  Fermi  and  his 
colleagues,  H.  L.  Anderson,  W.  H.  Zinn,  G.  Weil,  and  others, 
this  pile  has  served  as  a  prototype  unit  for  studies  of  thermal 
stability,  controls,  instruments,  and  shielding,  and  as  a  neutron 
source  for  materials  testing  and  neutron-physics  studies.  Further- 
more, it  has  proved  valuable  as  a  training  school  for  plant  oper- 
ators. More  recently  a  heavy- water  pile  (see  below)  has  been 
constructed  there. 

8.28.  The  first  Argonne  pile,  a  graphite-uranium  pile,  need 
not  be  described  in  detail.  The  materials  and  lattice  structure 
are  nearly  identical  to  those  which  were  used  for  the  original 

140  Plutonium  Problem  to  June  1945 

West  Stands  pile.  The  pile  is  a  cube;  it  is  surrounded  by  a  shield 
and  has  controls  and  safety  devices  somewhat  similar  to  those 
used  later  at  Clinton.  It  has  no  cooling  system  and  is  normally 
run  at  a  power  level  of  only  a  few  kilowatts.  It  has  occasionally 
been  run  at  high-power  levels  for  very  brief  periods.  Considering 
that  it  is  merely  a  reconstruction  of  the  first  chain-reacting  unit 
ever  built,  it  is  amazing  that  it  has  continued  in  operation  for 
more  than  two  years  without  developing  any  major  troubles. 

8.29.  One  of  the  most  valuable  uses  of  the  Argonne  pile  has 
been  the  measurement  of  neutron-absorption  cross  sections  of  a 
great  variety  of  elements  which  might  be  used  in  piles  as  struc- 
tural members,  etc.,  or  which  might  be  present  in  pile  materials 
as  impurities.  These  measurements  are  made  by  observing  the 
change  in  the  controls  necessary  to  make  k«ff  equal  to  1 .00  when 
a  known  amount  of  the  substance  under  study  is  inserted  at  a 
definite  position  in  the  pile.  The  results  obtained  were  usually 
expressed  in  terms  of  "danger  coefficients." 

8.30.  An  opening  at  the  top  of  the  pile  lets  out  a  very  uniform 
beam  of  thermal  neutrons  that  can  be  used  for  exponential-pile 
experiments,  for  direct  measurements  of  absorption  cross  sections, 
for  Wilson  cloud  chamber  studies,  etc. 

8.31.  An  interesting  phenomenon  occurring  at  the  top  of  the 
pile  is  the  production  of  a  beam  or  flow  of  "cold"  neutrons.  If  a 
sufficient  amount  of  graphite  is  interposed  between  the  upper 
surface  of  the  pile  and  an  observation  point  a  few  yards  above, 
the  neutron  energy  distribution  is  found  to  correspond  to  a 
temperature  much  lower  than  that  of  the  graphite.  This  is  pre- 
sumed to  be  the  result  of  a  preferential  transmission  by  the 
crystalline  graphite  of  the  slowest  ("coldest")  neutrons,  whose 
quantum-mechanical   wave-length   is    great    compared    to    the 
distance  between  successive  planes  in  the  graphite  crystals. 

8.32.  More  recently  a  pile  using  heavy  water  as  moderator 
was  constructed  in  the  Argonne  Laboratory.  The  very  high 
intensity  beam  of  neutrons  produced  by  this  pile  has  been  found 
well-suited  to  the  study  of  "neutron  optics,"  e.g.,  reflection  and 
refraction  of  neutron  beams  as  by  graphite. 

Plutonium  Problem  to  June  1945  141 

8.33.  A  constant  objective  of  the  Argonne  Laboratory  has  been 
a  better  understanding  of  nuclear  processes  in  uranium,  neptun- 
ium, and  plutonium.  Repeated  experiments  have  been  made  to 
improve  the  accuracy  of  constants  such  as  thermal-fission  cross 
sections  of  U-235,  U-238,  and  Pu-239,  probabilities  of  non-fission 
neutron  absorption  by  each  of  these  nuclei,  and  number  of  neu- 
trons emitted  per  fission. 


8.34.  In  Chapter  VI  we  mentioned  plans  for  a  "pilot"  plant 
for  production  of  plutonium  to  be  built  at  the  Clinton  site  in 
Tennessee.  By  January  1943,  the  plans  for  this  project  were  well 
along;  construction  was  started  soon  afterward.  M.  D.  Whitaker 
was  appointed  director  of  the  Clinton  Laboratories.  The  pilot- 
plant  plans  were  made  cooperatively  by  du  Pont  and  the  Metal- 
lurgical Laboratory;  construction  was  carried  out  by  du  Pont; 
plant  operation  was  maintained  by  the  University  of  Chicago 
as  part  of  the  Metallurgical  Project. 

8.35.  The  main  purposes  of  the  Clinton  plant  were  to  produce 
some  plutonium  and  to  serve  as  a  pilot  plant  for  chemical  separa- 
tion. As  regards  research,  the  emphasis  at  Clinton  was  on  chem- 
istry and  on  the  biological  effects  of  radiations.  A  large  laboratory 
was  provided  for  chemical  analysis,  for  research  on  purification 
methods,  for  fission-product  studies,  for  development  of  inter- 
mediate-scale  extraction   and   decontamination   processes,   etc. 
Later  a  "hot  laboratory,"  i.e.,  a  laboratory  for  remotely-con- 
trolled work  on  highly  radioactive  material,  was  provided.  There 
is  also  an  instrument  shop  and  laboratory  that  has  been  used  very 
actively.  There  are  facilities  for  both  clinical  and  experimental 
work  of  the  health  division,  which  has  been  very  active.  There  is 
a  small  physics  laboratory  in  which  some  important  work  was 
done  using  higher  neutron  intensities  than  were  available  at  the 
Argonne  Laboratory.  The  principal  installations  constructed  at 
the  Clinton  Laboratory  site  were  the  pile  and  the  separation 
plant;  these  are  briefly  described  below. 

142  Plutonium  Problem  to  June  1945 


8.36.  In  any  steadily  operating  pile  the  effective  multiplication 
factor  k  must  be  kept  at  1,  whatever  the  power  level.  The  best 
k^  that  had  been  observed  in  a  uranium-graphite  lattice  could 
not  be  achieved  in  a  practical  pile  because  of  neutron  leakage, 
cooling  system,  cylindrical  channels  for  the  uranium,  protective 
coating  on  the  uranium,  and  other  minor  factors.  Granted  air- 
cooling  and  a  maximum  safe  temperature  for  the  surface  of  the 
uranium,  a  size  of  pile  had  to  be  chosen  that  could  produce 
1 ,000  kw.  The  effective  k  would  go  down  with  rising  temperature 
but  not  sufficiently  to  be  a  determining  factor.  Though  a  sphere 
was  the  ideal  shape,  practical  considerations  recommended  a 
rectangular  block. 

8.37.  The  Clinton  pile  consists  of  a  cube  of  graphite  containing 
horizontal  channels  filled  with  uranium.  The  uranium  is  in  the 
form  of  metal  cylinders  protected  by  gas-tight  casings  of  alumi- 
num. The  uranium  cylinders  or  slugs  may  be  slid  into  the  chan- 
nels in  the  graphite;  space  is  left  to  permit  cooling  air  to  flow  past, 
and  to  permit  pushing  the  slugs  out  at  the  back  of  the  pile  when 
they  are  ready  for  processing.  Besides  the  channels  for  slugs  there 
are  various  other  holes  through  the  pile  for  control  rods,  instru- 
ments, etc. 

8.38.  The  Clinton  pile  was  considerably  larger  than  the  first 
pile  at  Chicago   (see  Chapter  VI).  More  important  than  the 
increased  size  of  the  Clinton  pile  were  its  cooling  system,  heavier 
shields,  and  means  for  changing  the  slugs.  The  production  goal 
of  the  Clinton  plant  was  set  at  a  figure  which  meant  that  the  pile 
should  operate  at  a  power  level  of  1,000  kw. 

8.39.  The  instrumentation  and  controls  are  identical  in  princi- 
ple to  those  of  the  first  pile.  Neutron  intensity  in  the  pile  is 
measured  by  a  BFs  ionization  chamber  and  is  controlled  by  boron 
steel  rods  that  can  be  moved  in  and  out  of  the  pile,  thereby  vary- 
ing the  fraction  of  neutrons  available  to  produce  fission. 

8.40.  In  spite  of  an  impressive  array  of  instruments  and  safety 
devices,  the  most  striking  feature  of  the  pile  is  the  simplicity  of 

Plutonium  Problem  to  June  1945  143 

operation.  Most  of  the  time  the  operators  have  nothing  to  do 
except  record  the  readings  of  various  instruments. 


8.41.  Here,  as  at  Hanford,  the  plutonium  processes  have  to 
be  carried  out  by  remote  control  and  behind  thick  shields.  The 
separation  equipment  is  housed  in  a  series  of  adjacent  cells  having 
heavy  concrete  walls.  These  cells  form  a  continuous  structure 
(canyon)  which  is  about  100  feet  long  and  is  two- thirds  buried  in 
the  ground.  Adjacent  to  this  canyon  are  the  control  rooms, 
analytical  laboratories,  and  a  laboratory  for  further  purification 
of  the  plutonium  after  it  has  been  decontaminated  to  the  point 
of  comparative  safety. 

8.42.  Uranium  slugs  that  have  been  exposed  in  the  pile  are 
transferred  under  water  to  the  first  of  these  cells  and  are  then 
dissolved.   Subsequent  operations   are   performed   by  pumping 
solutions  or  slurries  from  one  tank  or  centrifuge  to  another. 


8.43.  The  Clinton  pile  started  operating  on  November  4,  1943, 
and  within  a  few  days  was  brought  up  to  a  power  level  of  500  kw 
at  a  maximum  slug  surface  temperature  of  110°  C.  Improvements 
in  the  air  circulation  and  an  elevation  of  the  maximum  uranium 
surface  temperature  to  150°  C.  brought  the  power  level  up  to 
about  800  kw,  where  it  was  maintained  until  the  spring  of  1944. 
Starting  at  that  time,  a  change  was  made  in  the  distribution  of 
uranium,   the  change  being  designed  to  level  out  the  power 
distribution  in  the  pile  by  reducing  the  amount  of  metal  near  the 
center  relative  to  that  further  out  and  thereby  to  increase  the 
average  power  level  without  anywhere  attaining  too  high  a  tem- 
perature. At  the  same  time  improvements  were  realized  in  the 
sealing  of  the  slug  jackets,  making  it  possible  to  operate  the  pile 
at  higher  temperature.  As  a  result,  a  power  level  of  1,800  kw  was 
attained   in   May   1944;   this   was   further   increased   after   the 
installation  of  better  fans  in  June  1944. 

144  Plutonium  Problem  to  June  1945 

8.44.  Thus  the  pile  performance  of  June  1944  considerably 
exceeded  expectations.  In  ease  of  control,  steadiness  of  operation, 
and  absence  of  dangerous  radiation,   the  pile  has  been  most 
satisfactory.  There  have  been  very  few  failures  attributable  to 
mistakes  in  design  or  construction. 

8.45.  The   pile   itself  was  simple  both  in   principle  and  in 
practice.  Not  so  the  plutonium-separation  plant.  The  step  from 
the  first  chain-reacting  pile  to  the  Clinton  pile  was  reasonably 
predictable;  but  a  much  greater  and  more  uncertain  step  was 
required  in  the  case  of  the  separation  process,  for  the  Clinton 
separation  plant  was  designed  on  the  basis  of  experiments  using 
only  microgram  amounts  of  plutonium. 

8.46.  Nevertheless,  the  separation  process  worked!  The  first 
batch  of  slugs  from  the  pile  entered  the  separation  plant  on 
December  20,  1943.  By  the  end  of  January  1944,  metal  from  the 
pile  was  going  to  the  separation  plant  at  the  rate  of  %  ton  per 
day.  By  February  1 , 1 944, 1 90  mg  of  plutonium  had  been  delivered 
and  by  March  1, 1944,  several  grams  had  been  delivered.  Further- 
more, the  efficiency  of  recovery  at  the  very  start  was  about  50  per 
cent,  and  by  June  1 944  it  was  between  80  and  90  per  cent. 

8.47.  During  this  whole  period  there  was  a  large  group  of 
chemists   at   Clinton   working   on   improving   the   process   and 
developing  it  for  Hanford.  The  Hanford  problem  differed  from 
that  at  Clinton  in  that  much  higher  concentrations  of  plutonium 
were  expected.  Furthermore,  though  the  chemists  were  to  be 
congratulated  on  the  success  of  the  Clinton  plant,  the  process 
was  complicated  and  expensive.  Any  improvements  in  yield  or 
decontamination  or  in  general  simplification  were  very  much  to 
be  sought. 

8.48.  Besides  the  proving  of  the  pile  and  the  separation  plant 
and  the  production  of  several  grams  of  plutonium  for  experimental 
use  at  Chicago,  Clinton,  and  elsewhere,  the  Clinton  Laboratories 
have  been  invaluable  as  a  training  and  testing  center  for  Hanford, 
for  medical  experiments,  pile  studies,  purification  studies,  and 
physical  and  chemical  studies  of  plutonium  and  fission  products. 

8.49.  As  typical  of  the  kind  of  problems  tackled  there  and  at 

Plutonium  Problem  to  June  1945  145 

Chicago,  the  following  problems — listed  in  a  single  routine  report 
for  May  1944 — are  pertinent: 

Problems  Closed  Out  during  May  1944:  Search  for  New  Oxidizing 
Agent,  Effect  of  Radiation  on  Water  and  Aqueous  Solutions, 
Solubility  of  Plutonium  Peroxide,  Plutonium  Compounds  Suit- 
able for  Shipment,  Fission  Product  Distribution  in  Plant  Process 
Solutions,  Preliminary  Process  Design  for  Adsorption  Extraction, 
Adsorption  Semi- Works  Assistance,  Completion  of  Adsorption 
Process  Design. 

New  Problems  Assigned  during  May  1944:  New  Product  Analysis 
Method,  Effect  of  Radiation  on  Graphite,  Improvement  in 
Yield,  New  Pile  Explorations,  Waste  Uranium  Recovery, 
Monitoring  Stack  Gases,  Disposal  of  Active  Waste  Solutions, 
Spray  Cooling  of  X  Pile,  Assay  Training  Program,  Standardiza- 
tion of  Assay  Methods,  Development  of  Assay  Methods,  Shielded 
Apparatus  for  Process  Control  Assays,  Cloud  Chamber  Experi- 
ment, Alpha  Particles  from  U-235,  Radial  Product  Distribution, 
Diffraction  of  Neutrons. 


8.50.  It  is  beyond  the  scope  of  this  report  to  give  any  account 
of  the  construction  of  the  Hanford  Engineer  Works,  but  it  is  to 
be  hoped  that  the  full  story  of  this  extraordinary  enterprise  and 
the  companion  one,  the  Clinton  Engineer  Works,  will  be  pub- 
lished at  some  time  in  the  future.  The  Hanford  site  was  examined 
by  representatives  of  General  Groves  and  of  du  Pont  at  the  end 
of  1 942,  and  use  of  the  site  was  approved  by  General  Groves  after 
he  had  inspected  it  personally.  It  was  on  the  west  side  of  the 
Columbia  River  in  central  Washington  north  of  Pasco.  In  the 
early  months  of  1943  a  200-square-mile  tract  in  this  region  was 
acquired  by  the  government  (by  lease  or  purchase)  through  the 
Real  Estate  Division  of  the  Office  of  the  Chief  of  Engineers. 
Eventually  an  area  of  nearly  a  thousand  square  miles  was  brought 
under  government  control.  At  the  time  of  acquisition  of  the  land 
there  were  a  few  farms  and  two  small  villages,  Hanford  and 
Richland,  on  the  site,  which  was  otherwise  sage-brush  plains 

146  Plutonium  Problem  to  June  1945 

and  barren  hills.  On  the  6th  of  April,  1943,  ground  was  broken 
for  the  Hanford  construction  camp.  At  the  peak  of  activity  in 
1944,  this  camp  was  a  city  of  60,000  inhabitants,  the  fourth  largest 
city  in  the  state.  Now,  however,  the  camp  is  practically  deserted 
as  the  operating  crew  is  housed  at  Richland. 

8.51.  Work  was  begun  on  the  first  of  the  Hanford  production 
piles  on  June  7,  1943,  and  operation  of  the  first  pile  began  in 
September  1944.  The  site  was  originally  laid  out  for  five  piles, 
but  the  construction  of  only  three  has  been  undertaken.  Besides 
the   piles,    there   are,   of  course,   plutonium  separation   plants, 
pumping  stations  and  water-treatment  plants.  There  is  also  a 
low-power  chain-reacting  pile  for  material  testing.  Not  only  are 
the  piles  themselves  widely  spaced  for  safety — several  miles  apart 
— but  the  separation  plants  are  well  away  from  the  piles  and 
from  each  other.  All  three  piles  were  in  operation  by  the  summer 
of  1945. 


8.52.  No  one  who  lived  through  the  period  of  design  and  con- 
struction of  the  Hanford  plant  is  likely  to  forget  the  "canning" 
problem,  i.e.,  the  problem  of  sealing  the  uranium  slugs  in  pro- 
tective metal  jackets.  On  periodic  visits  to  Chicago  the  writer 
could  roughly  estimate  the  state  of  the  canning  problem  by  the 
atmosphere  of  gloom  or  joy  to  be  found  around  the  laboratory. 
It  was  definitely  not  a  simple  matter  to  find  a  sheath  that  would 
protect  uranium  from  water  corrosion,  would  keep  fission  products 
out  of  the  water,  would  transmit  heat  from  the  uranium  to  the 
water,  and  would  not  absorb  too  many  neutrons.  Yet  the  failure 
of  a  single  can  might  conceivably  require  shut-down  of  an  entire 
operating  pile. 

8.53.  Attempts  to  meet  the  stringent  requirements  involved 
experimental    work    on    electroplating    processes,    hot-dipping 
processes,  cementation-coating  processes,  corrosion-resistant  alloys 
of  uranium,   and   mechanical  jacketing  or   canning  processes. 
Mechanical  jackets  or  cans  of  thin  aluminum  were  feasible  from 
the  nuclear-physics  point  of  view  and  were  chosen  early  as  the 

Plutonium  Problem  to  June  1945  147 

most  likely  solution  of  the  problem.  But  the  problem  of  getting 
a  uniform,  heat-conducting  bond  between  the  uranium  and  the 
surrounding  aluminum,  and  the  problem  of  effecting  a  gas-tight 
closure  for  the  can  both  proved  very  troublesome.  Development 
of  alternative  methods  had  to  be  carried  along  up  to  the  last 
minute,  and  even  up  to  a  few  weeks  before  it  was  time  to  load 
the  uranium  slugs  into  the  pile  there  was  no  certainty  that  any 
of  the  processes  under  development  would  be  satisfactory.  A 
final  minor  but  apparently  important  modification  in  the  pre- 
ferred canning  process  was  adopted  in  October  1944,  after  the 
first  pile  had  begun  experimental  operation.  By  the  summer  of 
1945,  there  had  been  no  can  failure  reported. 


8.54.  During  the  fall  of  1944  and  the  early  months  of  1945 
the  second  and  third  Hanford  piles  were  finished  and  put  into 
operation,  as  were  the  additional  chemical  separation  plants. 
There  were,  of  course,  some  difficulties;  however,  none  of  the 
fears  expressed  as  to  canning  failure,  film  formation  in  the  water 
tubes,  or  radiation  effects  in  the  chemical  processes,  have  turned 
out  to  be  justified.  As  of  early  summer  1945  the  piles  are  operat- 
ing at  designed  power,  producing  plutonium,  and  heating  the 
Columbia  River.*  The  chemical  plants  are  separating  the  plu- 
tonium from  the  uranium  and  from  the  fission  products  with 
better  efficiency  than  had  been  anticipated.  The  finished  product 
is  being  delivered.  How  it  can  be  used  is  the  subject  of  Chapter 


8.55.  In  previous  chapters  there  have  been  references  to  the 
advantages  of  heavy  water  as  a  moderator.  It  is  more  effective 
than  graphite  in  slowing  down  neutrons  and  it  has  a  smaller 
neutron  absorption  than  graphite.  It  is  therefore  possible  to  build 

*  The  actual  rise  in  temperature  is  so  tiny  that  no  effect  on  fish  life  could 
be  expected.  To  make  doubly  sure,  this  expectation  was  confirmed  by  an 
elaborate  series  of  experiments. 

148  Plutonium  Problem  to  June  1945 

a  chain-reacting  unit  with  uranium  and  heavy  water  and  thereby 
to  attain  a  considerably  higher  multiplication  factor,  k,  and  a 
smaller  size  than  is  possible  with  graphite.  But  one  must  have 
the  heavy  water. 

8.56.  In   the  spring   of  1943   the   Metallurgical   Laboratory 
decided  to  increase  the  emphasis  on  experiments  and  calculations 
aimed  at  a  heavy- water  pile.  To  this  end  a  committee  was  set  up 
under  E.  Wigner,  a  group  under  H.  C.  Vernon  was  transferred 
from  Columbia  to  Chicago,  and  H.  D.  Smyth,  who  had  just 
become  associate  director  of  the  Laboratory,  was  asked  to  take 
general  charge. 

8.57.  The  first  function  of  this  group  was  to  consider  in  what 
way  heavy  water  could  best  be  used  to  insure  the  overall  success 
of  the  Metallurgical  Project,  taking  account  of  the  limited  produc- 
tion schedule  for  heavy  water  that  had  been  already  authorized. 

8.58.  It  became  apparent  that  the  production  schedule  was 
so  low  that  it  would  take  two  years  to  produce  enough  heavy 
water  to  "moderate"  a  fair-sized  pile  for  plutonium  production. 
On  the  other  hand,  there  might  be  enough  heavy  water  to  moder- 
ate a  small  "laboratory"  pile,  which  could  furnish  information 
that  might  be  valuable.  In  any  event,  during  the  summer  of  1943 
so  great  were  the  uncertainties  as  to  the  length  of  the  war  and 
as  to  the  success  of  the  other  parts  of  the  DSM  project  that  a 
complete  study  of  the  possibilities  of  heavy-water  piles  seemed 
desirable.  Either  the  heavy-water  production  schedule  might  be 
stepped  up  or  the  smaller,  experimental  pile  might  be  built.  An 
intensive  study  of  the  matter  was  made  during  the  summer  of 
1943  but  in  November  it  was  decided  to  curtail  the  program 
and  construction  was  limited  to  a  250-kw  pile  located  at  the 
Argonne  site. 


8.59.  Perhaps  the  most  striking  aspect  of  the  uranium  and 
heavy-water  pile  at  the  Argonne  is  its  small  size.  Even  with  its 
surrounding  shield  of  concrete  it  is  relatively  small  compared 
to  the  uranium-graphite  piles. 

Plutonium  Problem  to  June  1945  149 

8.60.  By  May  15,  1944,  the  Argonne  uranium  and  heavy-water 
pile  was  ready  for  test.  With  the  uranium  slugs  in  place,  it  was 
found  that  the  chain  reaction  in  the  pile  became  self  sustaining 
when  only  three  fifths  of  the  heavy  water  had  been  added.  The 
reactivity  of  the  pile  was  so  far  above  expectations  that  it  would 
have  been  beyond  the  capacity  of  the  control  rods  to  handle  if 
the  remainder  of  the  heavy  water  had  been  added.  To  meet  this 
unusual  and  pleasant  situation  some  of  the  uranium  was  removed 
and  extra  control  rods  were  added. 

8.61.  With  these  modifications  it  was  possible  to  fill  the  tank 
to  the  level  planned.  By  July  4,  1944,  W.  H.  Zinn  reported  that 
the  pile  was  running  satisfactorily  at  190  kw,  and  by  August  8, 
1 944,  he  reported  that  it  was  operating  at  300  kw. 

8.62.  In  general  the  characteristics  of  this  pile  differed  slightly 
from  those  of  comparable  graphite  piles.  This  pile  takes  several 
hours  to  reach  equilibrium.  It  shows  small  (less  than  1  per  cent) 
but  sudden  fluctuations   in  power  level,   probably  caused  by 
bubbles  in  the  water.  It  cannot  be  shut  down  as  completely  or 
as  rapidly  as  the  graphite  pile  because  of  the  tendency  of  delayed 
gamma   rays    to   produce    (from   the   heavy  water)    additional 
neutrons.  As  anticipated,  the  neutron  density  at  the  center  is 
high.  The  shields,  controls,  heat  exchanger,  etc.,  have  operated 


8.63.  The  major  objective  of  the  health  group  was  in  a  sense  a 
negative  one,  to  insure  that  no  one  concerned  suffered  serious 
injury  from  the  peculiar  hazards  of  the  enterprise.  Medical  case 
histories  of  persons  suffering  serious  injury  or  death  resulting 
from  radiation  were  emphatically  not  wanted.  The  success  of  the 
health  division  in  meeting  these  problems  was  remarkable.  Even 
in  the  research  group  where  control  is  most  difficult,  cases  show- 
ing even  temporary  bad  effects  were  extremely  rare.  Factors  of 
safety  used  in  plant  design  and  operation  are  so  great  that  the 
hazards  of  the  home  and  the  family  car  are  far  greater  for  the 
personnel  than  any  arising  from  the  plants. 

150  Plutonium  Problem  to  June  1945 

8.64.  To  achieve  its  objective  the  health  group  worked  along 
three  major  lines: 

(1)  Adoption  of  pre-employment  physical  examinations  and 
frequent    re-examinations,    particularly    of    those    exposed    to 

(2)  Setting  of  tolerance  standards  for  radiation  doses   and 
development  of  instruments  measuring  exposure  of  personnel; 
giving  advice  on  shielding,  etc. ;  continually  measuring  radiation 
intensities  at  various  locations  in  the  plants;  measuring  con- 
tamination of  clothes,  laboratory  desks,  waste  water,  the  atmos- 
phere, etc. 

(3)  Carrying  out  research  on  the  effects  of  direct  exposure  of 
persons  and  animals  to  various  types  of  radiation,  and  on  the 
effects  of  ingestion  and  inhalation  of  the  various  radioactive  or 
toxic  materials  such  as  fission  products,  plutonium  and  uranium. 


8.65.  The  white  blood-corpuscle  count  was  used  as  the  princi- 
pal criterion  as  to  whether  a  person  suffered  from  overexposure 
to  radiation.  A  number  of  cases  of  abnormally  low  counts  were 
observed  and  correlated  with  the  degree  of  overexposure.  Indi- 
viduals appreciably  affected  were  shifted  to  other  jobs  or  given 
brief  vacations;  none  has  shown  permanent  ill  effects. 

8.66.  At  the  same  time  it  was  recognized  that  the  white  blood- 
corpuscle  count  is  not  an  entirely  reliable  criterion.  Some  work 
on  animals  indicated  that  serious  damage  might  occur  before 
the  blood  count  gave  any  indication  of  danger.  Accordingly, 
more  elaborate  blood  tests  were  made  on  selected  individuals 
and  on  experimental  animals  in  the  hope  of  finding  a  test  that 
would  give  an  earlier  warning  of  impending  injury. 


8.67.  The  Health  Division  had  principal  responsibility  for  the 
development  of  pocket  meters  for  indicating  the  extent  of  ex- 
posure of  persons.  The  first  of  these  instruments  was  a  simple 

Plutonium  Problem  to  June  1945  151 

electroscope  about  the  size  and  shape  of  a  fountain  pen.  Such 
instruments  were  electrostatically  charged  at  the  start  of  each 
day  and  were  read  at  the  end  of  the  day.  The  degree  to  which 
they  became  discharged  indicated  the  total  amount  of  ionizing 
radiation  to  which  they  had  been  exposed.  Unfortunately  they 
were  none  too  rugged  and  reliable,  but  the  error  of  reading  was 
nearly  always  in  the  right  direction — i.e.,  in  the  direction  of  over- 
stating the  exposure.  At  an  early  date  the  practice  was  estab- 
lished of  issuing  two  of  these  pocket  meters  to  everyone  entering 
a  dangerous  area.  A  record  was  kept  of  the  readings  at  the  time 
of  issuance  and  also  when  the  meters  were  turned  in.  The  meters 
themselves  were  continually  although  gradually  improved.  The 
Health  Division  later  introduced  "film  badges,"  small  pieces  of 
film  worn  in  the  identification  badge,  the  films  being  periodically 
developed  and  examined  for  radiation  blackening.  These  instru- 
ments for  individuals  such  as  the  pocket  meter  and  film  badge 
were  extra  and  probably  unnecessary  precautions.  In  permanent 
installations  the  shielding  alone  normally  affords  complete  safety. 
Its  effect  is  under  frequent  survey  by  either  permanently  installed 
or  portable  instruments. 

8.68.  The  Health  Division  cooperated  with  the  Physics  Division 
in  the  development  and  use  of  various  other  instruments.  There 
was  "Sneezy"  for  measuring  the  concentration  of  radioactive 
dust  in  the  air  and  "Pluto"  for  measuring  a-emitting  contamina- 
tion  (usually  plutonium)  of  laboratory  desks  and  equipment. 
Counters  were  used  to  check  the  contamination  of  laboratory 
coats  before  and  after  the  coats  were  laundered.  At  the  exit  gates 
of  certain  laboratories   concealed  counters  sounded  an  alarm 
when  someone  passed  whose  clothing,  skin  or  hair  was  con- 
taminated. In  addition,  routine  inspections  of  laboratory  areas 
were  made. 

8.69.  One  of  the  studies  made  involved  meteorology.  It  be- 
came essential  to  know  whether  the  stack  gases  (at  Clinton  and 
at  Hanford)  would  be  likely  to  spread  radioactive  fission  products 
in  dangerous  concentrations.  Since  the  behavior  of  these  gases 
is  very  dependent  on  the  weather,  studies  were  made  at  both 

152  Plutonium  Problem  to  June  1945 

sites  over  a  period  of  many  months,  and  satisfactory  stack  opera- 
tion was  specified. 


8.70.  Since  both  the  scale  and  the  variety  of  the  radiation 
hazards  in  this  enterprise  were  unprecedented,  all  reasonable 
precautions  were  taken;  but  no  sure  means  were  at  hand  for 
determining  the  adequacy  of  the  precautions.  It  was  essential  to 
supplement  previous  knowledge  as  completely  as  possible.  For 
this  purpose,  an  extensive  program  of  animal  experimentation 
was  carried  out  along  three  main  lines:  (1)  exposure  to  neutron, 
alpha,  beta  and  gamma  radiation;   (2)  ingestion  of  uranium, 
plutonium   and   fission   products;    (3)    inhalation   of  uranium, 
plutonium  and  fission  products.  Under  the  general  direction  of 
Dr.  Stone  these  experiments  were  carried  out  at  Chicago,  Clinton 
and  the  University  of  California  principally  by  Dr.  Cole  and  Dr. 
Hamilton.  Extensive  and  valuable  results  were  obtained. 


8.71.  Both  space  and  security  restrictions  prevent  a  detailed 
report  on  the  work  of  the  laboratories  and  plants  concerned  with 
plutonium  production. 

8.72.  Two  types  of  neutron  absorption  are  fundamental  to  the 
operation  of  the  plant:  one,  neutron  absorption  in  U-235  result- 
ing in  fission,  maintains  the  chain  reaction  as  a  source  of  neutrons; 
the  other,  neutron  absorption  in  U-238  leads  to  the  formation  of 
plutonium,  the  desired  product. 

8.73.  The  course  of  a  nuclear  chain  reaction  in  a  graphite- 
moderated  heterogeneous  pile  can  be  described  by  following  a 
single  generation  of  neutrons.   The  original  fast  neutrons  are 
slightly  increased  in  number  by  fast  fission,  reduced  by  resonance 
absorption  in  U-238  and  further  reduced  by  absorption  at  thermal 
energies  in  graphite  and  other  materials  and  by  escape;  the 
remaining  neutrons,  which  have  been  slowed  in  the  graphite, 
cause   fission   in   U-235,   producing  a  new   generation   of  fast 
neutrons  similar  to  the  previous  generation. 

Plutonium  Problem  to  June  7945  153 

8.74.  The  product,  plutonium,  must  be  separated  by  chemical 
processes  from  a  comparable  quantity  of  fission  products  and  a 
much  larger  quantity  of  uranium.  Of  several  possible  separation 
processes  the  one  chosen  consists  of  a  series  of  reactions  including 
precipitating  with  carriers,  dissolving,  oxidizing  and  reducing. 

8.75.  The  chain  reaction  was  studied  at  low  power  at  the 
Argonne  Laboratory  beginning  early  in  1943.  Both  chain  reaction 
and   chemical   separation   processes   were   investigated   at   the 
Clinton   Laboratories   beginning   in   November   1943,   and   an 
appreciable  amount  of  plutonium  was  produced  there. 

8.76.  Construction  of  the  main  production  plant  at  Hanford, 
Washington,  was  begun  in  1943  and  the  first  large  pile  went  into 
operation  in  September  1 944.  The  entire  plant  was  in  operation 
by  the  summer  of  1945  with  all  chain-reacting  piles  and  chemical- 
separation  plants  performing  better  than  had  been  anticipated. 

8.77.  Extensive  studies  were  made  on  the  use  of  heavy  water 
as  a  moderator  and  an  experimental  pile  containing  heavy  water 
was  built  at  the  Argonne  Laboratory.  Plans  for  a  production 
plant  using  heavy  water  were  given  up. 

8.78.  The  Health  Division  was  active  along  three  main  lines: 
(1)  medical  examination  of  personnel;  (2)  advice  on  radiation 
hazards  and  constant  check  on  working  conditions;  (3)  research 
on  the  effects  of  radiation.  The  careful  planning  and  exhaustive 
research  work  of  this  division  have  resulted  in  an  outstanding 
health  record  at  Hanford  and  elsewhere  in  the  project. 



9.1.  The  possibility  of  producing  an  atomic  bomb  of  U-235 
was  recognized  before  plutonium  was  discovered.  Because  it  was 
appreciated  at  an  early  date  that  the  separation  of  the  uranium 
isotopes  would  be  a  direct  and  major  step  toward  making  such  a 
bomb,  methods  of  separating  uranium  isotopes  have  been  under 
scrutiny  for  at  least  six  years.  Nor  was  attention  confined  to 
uranium  since  it  was  realized  that  the  separation  of  deuterium 
was  also  of  great  importance.  In  the  present  chapter  the  general 
problems  of  isotope  separation  will  be  discussed;  later  chapters 
will  take  up  the  specific  application  of  various  processes. 


9.2.  By  definition,  the  isotopes  of  an  element  differ  in  mass  but 
not  in  chemical  properties.  More  precisely,  although  the  nuclear 
masses  and  structures  differ,  the  nuclear  charges  are  identical 
and  therefore  the  external  electronic  structures  are  practically 
identical.  For  most  practical  purposes,  therefore,  the  isotopes  of 
an  element  are  separable  only  by  processes  depending  on  the 
nuclear  mass. 

9.3.  It  is  well  known  that  the  molecules  of  a  gas  or  liquid  are 
in  continual  motion  and  that  their  average  kinetic  energy  depends 
only  on  the  temperature,  not  on  the  chemical  properties  of  the 
molecules.  Thus  in  a  gas  made  up  of  a  mixture  of  two  isotopes 
the  average  kinetic  energy  of  the  light  molecules  and  of  the  heavy 
ones  is  the  same.  Since  the  kinetic  energy  of  a  molecule  is  J^  mv2, 
where  m  is  the  mass  and  v  the  speed  of  the  molecule,  it  is  appar- 
ent that  on  the  average  the  speed  of  a  lighter  molecule  must  be 


The  Separation  of  Isotopes  155 

greater  than  that  of  a  heavier  molecule.  Therefore,  at  least  in 
principle,  any  process  depending  on  the  average  speed  of  mole- 
cules can  be  used  to  separate  isotopes.  Unfortunately,  the  average 
speed  is  inversely  proportional  to  the  square  root  of  the  mass  so 
that  the  difference  is  very  small  for  the  gaseous  compounds  of 
the  uranium  isotopes.  Also,  although  the  average  speeds  differ,  the 
ranges  of  speed  show  considerable  overlap.  In  the  case  of  the  gas 
uranium  hexafluoride,  for  example,  over  49  per  cent  of  the  light 
molecules  have  speeds  as  low  as  those  of  50  per  cent  of  the  heavy 

9.4.  Obviously  there  is  no  feasible  way  of  applying  mechanical 
forces  directly  to  molecules  individually;  they  cannot  be  poked 
with  a  stick  or  pulled  with  a  string.  But  they  are  subject  to  gravi- 
tational fields  and,  if  ionized,  may  be  affected  by  electric  and 
magnetic  fields.  Gravitational  forces  are,  of  course,  proportional 
to  the  mass.  In  a  very  high  vacuum  U-235  atoms  and  U-238  atoms 
would  fall  with  the  same  acceleration,  but  just  as  a  feather  and 
a  stone  fall  at  very  different  rates  in  air  where  there  are  frictional 
forces  resisting  motion,  there  may  be  conditions  under  which  a 
combination  of  gravitational  and  opposing  intermolecular  forces 
will   tend   to   move  heavy  atoms   differently  from   light  ones. 
Electric  and  magnetic  fields   are  more  easily  controlled  than 
gravitational  fields  or  "pseudogravitational"  fields  (i.e.,  centrif- 
ugal-force fields)   and   are  very  effective  in  separating  ions  of 
differing  masses. 

9.5.  Besides  gravitational  or  electromagnetic  forces,  there  are, 
of  course,  interatomic  and  intermolecular  forces.  These  forces 
govern  the  interaction  of  molecules  and  thus  affect  the  rates  of 
chemical  reactions,  evaporation  processes,  etc.  In  general,  such 
forces  will  depend  on  the  outer  electrons  of  the  molecules  and  not 
on  the  nuclear  masses.  However,  whenever  the  forces  between 
separated  atoms   or  molecules   lead   to   the  formation  of  new 
molecules,  a  mass  effect  (usually  very  small)  does  appear.  In 
accordance  with  quantum-mechanical  laws,   the  energy  levels 
of  the  molecules  are  slightly  altered,  and  differently  for  each 
isotope.  Such  effects  do  slightly  alter  the  behavior  of  two  isotopes 

156  The  Separation  of  Isotopes 

in  certain  chemical  reactions,  as  we  shall  see,  although  the  dif- 
ference in  behavior  is  far  smaller  than  the  familiar  differences  of 
chemical  behavior  between  one  element  and  another. 

9.6.  These,  then,  are  the  principal  factors  that  may  have  to  be 
considered  in  devising  a  separation  process:  equality  of  average 
thermal  kinetic  energy  of  molecules  at  a  given  temperature, 
gravitational  or  centrifugal  effects  proportional  to  the  molecular 
masses,  electric  or  magnetic  forces  affecting  ionized  molecules, 
and    interatomic    or    intermolecular   forces.    In   some    isotope- 
separation  processes  only  one  of  these  effects  is  involved  and  the 
overall  rate  of  separation  can  be  predicted.  In  other  isotope- 
separation  processes  a  number  of  these  effects  occur  simultane- 
ously so  that  prediction  becomes  difficult. 


9.7.  Before  discussing  particular  processes  suitable  for  isotope 
separation,  we  should  know  what  is  wanted.  The  major  criteria 
to  be  used  in  judging  an  isotope-separation  process  are  as  follows. 


9.8.  The  separation  factor,  sometimes  known  as  the  enrich- 
ment or  fractionating  factor  of  a  process,  is  the  ratio  of  the  relative 
concentration  of  the  desired  isotope  after  processing  to  its  relative 
concentration  before  processing.  Defined  more  precisely:  if,  before 
the  processing,  the  numbers  of  atoms  of  the  isotopes  of  mass 
number  mi  and  m2  are  ni  and  n2  respectively  (per  gram  of  the 
isotope  mixture)  and  if,  after  the  processing,  the  corresponding 
numbers  are  n^  and  n'2,  then  the  separation  factor  is: 


This  definition  may  be  applied  to  one  stage  of  a  separation  plant 
or  to  an  entire  plant  consisting  of  many  stages.  We  are  usually 
interested  either  in  the  "single  stage"  separation  factor  or  in  the 
"overall"  separation  factor  of  the  whole  process.  If  r  is  only 
slightly  greater  than  unity,  as  is  often  the  case  for  a  single  stage, 

The  Separation  of  Isotopes  1 57 

the  number  r  —  1  is  sometimes  more  useful  than  r.  The  quantity 
r  —  1  is  called  the  enrichment  factor.  In  natural  uranium 
mi  =  235,  mz  =  238,  and  ni/n2  =  K40  approximately,  but 
in  90  per  cent  U-235,  n\/n'2  —  %.  Consequently  in  a  process 
producing  90  per  cent  U-235  from  natural  uranium  the  overall 
value  of  r  must  be  about  1,260. 


9.9.  In  nearly  every  process  a  high  separation  factor  means  a 
low  yield,  a  fact  that  calls  for  continual  compromise.  Unless 
indication  is  given  to  the  contrary,  we  shall  state  yields  in  terms 
of  U-235.  Thus  a  separation  device  with  a  separation  factor  of 
2 — that  is,  nj/n'a  =  Mo — and  a  yield  of  one  gram  a  day  is  one  that, 
starting  from  natural  uranium,  produces,  in  one  day,  material 
consisting  of  1  gram  of  U-235  mixed  with  70  grams  of  U-238. 


9.10.  The  total  amount  of  material  tied  up  in  a  separation 
plant  is  called  the  "hold-up."  The  hold-up  may  be  very  large 
in  a  plant  consisting  of  many  stages. 


9.11.  In  a  separation  plant  having  large  hold-up,  a  long  time 
— perhaps  weeks  or  months — is  needed  for  steady  operating  con- 
ditions to  be  attained.  In  estimating  time  schedules  this  "start- 
up"   or   "equilibrium"    time   must   be   added   to   the   time   of 
construction  of  the  plant. 


9.12.  If  a  certain  quantity  of  raw  material  is  fed  into  a  separa- 
tion plant,  some  of  the  material  will  be  enriched,  some  impover- 
ished, some  unchanged.  Parts  of  each  of  these  three  fractions  will 
be  lost  and  parts  recovered.  The  importance  of  highly  efficient 
recovery  of  the  enriched  material  is  obvious.  In  certain  processes 
the  amount  of  unchanged  material  is  negligible,  but  in  others, 
notably  in  the  electromagnetic  method  to  be  described  below,  it 

158  The  Separation  of  Isotopes 

is  the  largest  fraction  and  consequently  the  efficiency  with  which 
it  can  be  recovered  for  recycling  is  very  important.  The  impor- 
tance of  recovery  of  impoverished  material  varies  widely,  depend- 
ing very  much  on  the  degree  of  impoverishment.  Thus  in  general 
there  are  many  different  efficiencies  to  be  considered. 


9.13.  As  in  all  parts  of  the  uranium  project,  cost  in  time  was 
more  important  than  cost  in  money.  Consequently  a  number  of 
large-scale  separation  plants  for  U-235  and  deuterium  were  built 
at  costs  greater  than  would  have  been  required  if  construction 
could  have  been  delayed  for  several  months  or  years  until  more 
ideal  processes  were  worked  out. 



9.14.  As  long  ago  as  1896  Lord  Rayleigh  showed  that  a  mixture 
of  two  gases  of  different  atomic  weight  could  be  partly  separated 
by  allowing  some  of  it  to  diffuse  through  a  porous  barrier  into  an 
evacuated  space.   Because   of  their   higher   average  speed   the 
molecules  of  the  light  gas  diffuse  through  the  barrier  faster  so 
that  the  gas  which  has  passed  through  the  barrier   (i.e.,   the 
"diffusate")  is  enriched  in  the  lighter  constituent  and  the  residual 
gas  which  has  not  passed  through  the  barrier  is  impoverished 
in  the  lighter  constituent.  The  gas  most  highly  enriched  in  the 
lighter  constituent  is  the  so-called  "instantaneous  diffusate";  it 
is  the  part  that  diffuses  before  the  impoverishment  of  the  residue 
has  become  appreciable.  If  the  diffusion  process  is  continued 
until  nearly  all  the  gas  has  passed  through  the  barrier,  the  average 
enrichment  of  the  diffusate  naturally  diminishes.   In  the  next 
chapter    we    shall     consider     these    phenomena    more    fully. 
Here  we  shall  merely  point  out  that,  on  the  assumption  that  the 
diffusion  rates  are  inversely  proportional  to  the  square  roots  of 
the  molecular  weights  the  separation  factor  for  the  instantaneous 
diffusate,  called  the  "ideal  separation  factor"  a,  is  given  by 

The  Separation  of  Isotopes  159 


where  MI  is  the  molecular  weight  of  the  lighter  gas  and  M2  that 
of  the  heavier.  Applying  this  formula  to  the  case  of  uranium  will 
illustrate  the  magnitude  of  the  separation  problem.  Since  uranium 
itself  is  not  a  gas,  some  gaseous  compound  of  uranium  must  be 
used.  The  only  one  obviously  suitable  is  uranium  hexafluoride, 
UFe,  which  has  a  vapor  pressure  of  one  atmosphere  at  a  tem- 
perature of  56°  C.  Since  fluorine  has  only  one  isotope,  the  two 
important  uranium  hexafluorides  are  U236F6  and  U238F6;  their 
molecular  weights  are  349  and  352.  Thus,  if  a  small  fraction  of  a 
quantity  of  uranium  hexafluoride  is  allowed  to  diffuse  through  a 
porous  barrier,  the  diffusate  will  be  enriched  in  U235Fe  by  a  factor 

=  1.0043 

which  is  a  long  way  from  the  1,260  required  (see  paragraph  9.8.). 

9.15.  Such  calculations  might  make  it  seem  hopeless  to  sepa- 
rate isotopes    (except,   perhaps,    the  isotopes   of  hydrogen)   by 
diffusion  processes.  Actually,  however,  such  methods  may  be 
used  successfully  —  even  for  uranium.  It  was  the  gaseous  diffusion 
method  that  F.  W.  Aston  used  in  the  first  partial  separation  of 
isotopes   (actually  the  isotopes  of  neon).  Later  G.  Hertz  and 
others,  by  operating  multi-stage  recycling  diffusion  units,  were 
able  to  get  practically  complete  separation  of  the  neon  isotopes. 
Since  the  multiple-stage  recycling  system  is  necessary  for  nearly 
all  separation  methods,  it  will  be  described  in  some  detail  immedi- 
ately following  introductory  remarks  on  the  various  methods  to 
which  it  is  pertinent. 


9.16.  The  separation  of  compounds  of  different  boiling  points, 
i.e.,  different  vapor  pressures,  by  distillation  is  a  familiar  indus- 
trial  process.   The  separation  of  alcohol  and  water   (between 
which  the  difference  in  boiling  point  is  in  the  neighborhood  of 

160  The  Separation  of  Isotopes 

20°  C.)  is  commonly  carried  out  in  a  simple  still  using  but  a 
single  evaporator  and  condenser.  The  condensed  material  (con- 
densate)  may  be  collected  and  redistilled  a  number  of  times  if 
necessary.  For  the  separation  of  compounds  of  very  nearly  the 
same  boiling  point  it  would  be  too  laborious  to  carry  out  the 
necessary  number  of  successive  evaporations  and  condensations 
as  separate  operations.  Instead,  a  continuous  separation  is 
carried  out  in  a  fractionating  tower.  Essentially  the  purpose  of 
a  fractionating  tower  is  to  produce  an  upward-directed  stream 
of  vapor  and  a  downward-directed  stream  of  liquid,  the  two 
streams  being  in  intimate  contact  and  constantly  exchanging 
molecules.  The  molecules  of  the  fraction  having  the  lower  boiling 
point  have  a  relatively  greater  tendency  to  get  into  the  vapor 
stream  and  vice  versa.  Such  counter-current  distillation  methods 
can  be  applied  to  the  separation  of  light  and  heavy  water,  which 
differ  in  boiling  point  by  1 .4°  C. 


9.17.  The  method  of  counter  current  flow  is  useful  not  only  in 
two-phase  (liquid-gas)  distillation  processes,  but  also  in  other 
separation  processes  such  as  those  involving  diffusion  resulting 
from  temperature  variations  (gradients)  within  one-phase  systems 
or  from  centrifugal  forces.  The  countercurrents  may  consist  of 
two  gases,  two  liquids,  or  one  gas  and  one  liquid. 


9.18.  We  have  pointed  out  that  gravitational  separation  of 
two  isotopes  might  occur  since  the  gravitational  forces  tending 
to  move  the  molecules  downward  are  proportional  to  the  mole- 
cular weights,  and  the  intermolecular  forces  tending  to  resist 
the  downward  motion  depend  on  the  electronic  configuration, 
not  on  the  molecular  weights.  Since  the  centrifuge  is  essentially 
a  method  of  applying  pseudogravitational  forces  of  large  magni- 
tude, it  was  early  considered  as  a  method  for  separating  isotopes. 
However,   the  first  experiments  with  centrifuges  failed.   Later 

The  Separation  of  Isotopes  161 

development  of  the  high  speed  centrifuge  by  J.  W.  Beams  and 
others  led  to  success.  H.  C.  Urey  suggested  the  use  of  tall  cylin- 
drical centrifuges  with  countercurrent  flow;  such  centrifuges  have 
been  developed  successfully. 

9.19.  In  such  a  countercurrent  centrifuge  there  is  a  downward 
flow  of  vapor  in  the  outer  part  of  the  rotating  cylinder  and  an 
upward  flow  of  vapor  in  the  central  or  axial  region.  Across  the 
interface  region  between  the  two  currents  there  is  a  constant 
diffusion  of  both  types  of  molecules  from  one  current  to  the  other, 
but  the  radial  force  field  of  the  centrifuge  acts  more  strongly  on 
the  heavy  molecules  than  on  the  light  ones  so  that  the  concentra- 
tion of  heavy  ones  increases  in  the  peripheral  region  and  decreases 
in  the  axial  region,  and  vice  versa  for  the  lighter  molecules. 

9.20.  The  great  appeal  of  the  centrifuge  in  the  separation  of 
heavy  isotopes  like  uranium  is  that  the  separation  factor  depends 
on  the  difference  between  the  masses  of  the  two  isotopes,  not  on 
the  square  root  of  the  ratio  of  the  masses  as  in  diffusion  methods. 


9.21.  The  kinetic  theory  of  gases  predicts  the  extent  of  the 
differences  in  the  rates  of  diffusion  of  gases  of  different  molecular 
weights.  The  possibility  of  accomplishing  practical  separation  of 
isotopes  by  thermal  diffusion  was  first  suggested  by  theoretical 
studies  of  the  details  of  molecular  collisions  and  of  the  forces 
between  molecules.  Such  studies  made  by  Enskog  and  by  Chap- 
man before  1920  suggested  that  if  there  were  a  temperature 
gradient  in  a  mixed  gas  there  would  be  a  tendency  for  one  type 
of  molecule  to  concentrate  in  the  cold  region  and  the  other  in 
the  hot  region.  This  tendency  depends  not  only  on  the  molecular 
weights  but  also  on  the  forces  between  the  molecules.  If  the  gas 
is  a  mixture  of  two  isotopes,  the  heavier  isotope  may  accumulate 
at  the  hot  region  or  the  cold  region  or  not  at  all,  depending  on 
the  nature  of  the  intermolecular  forces.  In  fact,  the  direction  of 
separation  may  reverse  as  the  temperature  or  relative  concen- 
tration is  changed. 

1 62  The  Separation  of  Isotopes 

9.22.  Such  thermal  diffusion  effects  were  first  used  to  separate 
isotopes  by  H.  Clusius  and  G.  Dickel  in  Germany  in  1938.  They 
built  a  vertical  tube  containing  a  heated  wire  stretched  along 
the  axis  of  the  tube  and  producing  a  temperature  difference  of 
about  600°  C.  between  the  axis  and  the  periphery.  The  effect  was 
twofold.  In  the  first  place,  the  heavy  isotopes  (in  the  substances 
they  studied)  became  concentrated  near  the  cool  outer  wall,  and 
in  the  second  place,  the  cool  gas  on  the  outside  tended  to  sink 
while  the  hot  gas  at  the  axis  tended  to  rise.  Thus  thermal  con- 
vection set  up   a   countercurrent  flow,    and   thermal   diffusion 
caused  the  preferential  flow  of  the  heavy  molecules  outward 
across  the  interface  between  the  two  currents. 

9.23.  The  theory  of  thermal  diffusion  in   gases   is  intricate 
enough;  that  of  thermal  diffusion  in  liquids  is  practically  impos- 
sible. A  separation  effect  does  exist,  however,  and  has  been  used 
successfully  to  separate  the  light  and  heavy  uranium  hexafluorides. 


9.24.  In  the  introduction  to  this  chapter  we  pointed  out  that 
there  was  some  reason  to  hope  that  isotope  separation  might  be 
accomplished  by  ordinary  chemical  reactions.  It  has  in  fact  been 
found  that  in  simple  exchange  reactions  between  compounds  of 
two  different  isotopes  the  so-called  equilibrium  constant  is  not 
exactly  one,  and  thus  that  in  reactions  of  this  type  separation 
can  occur.  For  example,  in  the  catalytic  exchange  of  hydrogen 
atoms   between   hydrogen  gas   and  water,   the  water  contains 
between  three  and  four  times  as  great  a  concentration  of  deu- 
terium as  the  hydrogen  gas  in  equilibrium  with  it.  With  hydrogen 
and  water  vapor  the  effect  is  of  the  same  general  type  but  equilib- 
rium is  more  rapidly  established.   It  is  possible  to  adapt  this 
method  to  a  continuous  countercurrent  flow  arrangement  like 
that  used  in  distillation,  and  such  arrangements  are  actually  in 
use  for  production  of  heavy  water.  The  general  method  is  well 
understood,  and  the  separation  effects  are  known  to  decrease  in 
general  with  increasing  molecular  weight,  so  that  there  is  but  a 

The  Separation  of  Isotopes  163 

small  chance  of  applying  this  method  successfully  to  heavy  iso- 
topes like  uranium. 


9.25.  The  electrolysis  method  of  separating  isotopes  resulted 
from  the  discovery  that  the  water  contained  in  electrolytic  cells 
used  in  the  regular  commercial  production  of  hydrogen  and 
oxygen  has  an  increased  concentration  of  heavy  water  molecules. 
A  full  explanation  of  the  effect  has  not  yet  been  worked  out. 
Before  the  war  practically  the  entire  production  of  heavy  hydro- 
gen was  by  the  electrolysis  method.  By  far  the  greatest  production 
was  in  Norway,  but  enough  for  many  experimental  purposes  had 
been  made  in  the  United  States. 


9.26.  The  six  methods  of  isotope  separation  we  have  described 
so  far  (diffusion,  distillation,  centrifugation,  thermal  diffusion, 
exchange  reactions,  and  electrolysis)  have  all  been  tried  with  some 
degree  of  success  on  either  uranium  or  hydrogen  or  both.  Each 
of  these  methods  depends  on  small  differences  in  the  average 
behavior  of  the  molecules  of  different  isotopes.  Because  an  average 
is  by  definition  a  statistical  matter,  all  such  methods  depending 
basically  on  average  behavior  are  called  statistical  methods. 

9.27.  With  respect  to  the  criteria  set  up  for  judging  separation 
processes  the  six  statistical  methods  are  rather  similar.  In  every 
case  the  separation  factor  is  small  so  that  many  successive  stages 
of  separation  are  required.  In  most  cases  relatively  large  quantities 
of  material  can  be  handled  in  plants  of  moderate  size.  The  hold- 
up and  starting-time  values  vary  considerably  but  are  usually  high. 
The  similarity  of  the  six  methods  renders  it  inadvisable  to  make 
final  choice  of  method  without  first  studying  in  detail  the  particu- 
lar isotope,  production  rate,  etc.,  wanted.   Exchange  reaction 
and  electrolysis  methods  are  probably  unsuitable  in  the  case  of 
uranium,  and  no  distillation  scheme  for  uranium  has  survived. 
All  of  the  other  three  methods  have  been  developed  with  varying 
degrees  of  success  for  uranium,  but  are  not  used  for  hydrogen. 


The  Separation  of  Isotopes 


9.28.  The  existence  of  non-radioactive  isotopes  was  first 
demonstrated  during  the  study  of  the  behavior  of  ionized  gas 
molecules  moving  through  electric  and  magnetic  fields.  It  is  just 







such  fields  that  form  the  basis  of  the  so-called  mass  spectrographic 
or  electromagnetic  method  of  separating  isotopes.  This  method 
is  the  best  available  for  determining  the  relative  abundance  of 
many  types  of  isotope.  The  method  is  used  constantly  in  checking 
the  results  of  the  uranium  isotope  separation  methods  we  have 
already  described.  The  reason  the  method  is  so  valuable  is  that 
it  can  readily  effect  almost  complete  separation  of  the  isotopes 
very  rapidly  and  with  small  hold-up  and  short  start-up  time.  If 

The  Separation  of  Isotopes  165 

this  is  so,  it  may  well  be  asked  why  any  other  method  of  separa- 
tion is  considered.  The  answer  is  that  an  ordinary  mass  spectro- 
graph  can  handle  only  very  minute  quantities  of  material,  usually 
of  the  order  of  fractions  of  a  microgram  per  hour. 

9.29.  To  understand  the  reasons  for  this  limitation  in  the  yield, 
we  shall  outline  the  principle  of  operation  of  a  simple  type  of 
mass  spectrograph  first  used  by  A.  J.  Dempster  in  1918.  Such  an 
instrument  is  illustrated  schematically  in  the  drawing  on  p.  164. 
The  gaseous  compound  to  be  separated  is  introduced  in  the 
ion  source,  where  some  of  its  molecules  are  ionized  in  an  electric 
discharge.  Some  of  these  ions  go  through  the  slit  SL  Between 
Si  and  82  they  are  accelerated  by  an  electric  field  which  gives 
them  all  practically  the  same  kinetic  energy,  thousands  of  times 
greater  than  their  average  thermal  energy.  Since  they  now  all 
have  practically  the  same  kinetic  energy,  the  lighter  ions  must 
have  less  momenta  than  the  heavy  ones.  Entering  the  magnetic 
field  at  the  slit  82  all  the  ions  will  move  perpendicular  to  the 
magnetic  field  in  semi-circular  paths  of  radii  proportional  to 
their  momenta.  Therefore  the  light  ions  will  move  in  smaller 
semicircles  than  the  heavy,  and  with  proper  positioning  of  the 
collector,  only  the  light  ions  will  be  collected. 

9.30.  Postponing    detailed    discussion    of  such   a   separation 
device,  we  may  point  out  the  principal  considerations  that  limit 
the  amount  of  material  that  passes  through  it.  They  are  three- 
fold: First,  it  is  difficult  to  produce  large  quantities  of  gaseous 
ions.  Second,  a  sharply  limited  ion  beam  is  usually  employed  (as 
in  the  case  shown)  so  that  only  a  fraction  of  the  ions  produced 
are  used.  Third,  too  great  densities  of  ions  in  a  beam  can  cause 
space-charge  effects  which  interfere  with  the  separating  action. 
Electromagnetic  methods  developed  before  1941  had  very  high 
separation  factors  but  very  low  yields  and  efficiencies.  These  were 
the  reasons  which — before  the  summer  of  1941 — led  the  Uranium 
Committee  to  exclude  such  methods  for  large-scale  separation 
of  U-235.  (See  Paragraph  4:31.)  Since  that  time  it  has  been  shown 
that  the  limitations  are  not  insuperable.  In  fact,  the  first  appreci- 
able-size samples  of  pure  U-235  were  produced  by  an  electro- 
magnetic separator,  as  will  be  described  in  a  later  chapter. 

1 66  The  Separation  of  Isotopes 


9.31.  In  addition  to  the  isotope-separation  methods  described 
above,  several  other  methods  have  been  tried.  These  include  the 
ionic  mobility  method,  which,  as  the  name  implies,  depends  on 
the  following  fact:  In  an  electrolytic  solution  two  ions  which  are 
chemically  identical  but  of  different  mass  progress  through  the 
solution  at  different  rates  under  the  action  of  an  electric  field. 
However,   the  difference  of  mobility  will  be  small  and  easily 
obscured  by  disturbing  effects.  A.  K.  Brewer  of  the  Bureau  of 
Standards  reported  that  he  was  able  to  separate  the  isotopes  of 
potassium  by  this  method.  Brewer  also  obtained  some  interesting 
results  with  an  evaporation  method.  Two  novel  electromagnetic 
methods,  the  isotron  and  the  ionic  centrifuge,  are  described  in 
Chapter  XI.  The  isotron  produced  a  number  of  fair-size  samples 
of  partly  separated  uranium.  The  ionic  centrifuge  also  produced 
some  uranium  samples  showing  separation,  but  its  action  was 


9.32.  In  all  the  statistical  methods  of  separating  isotopes  many 
successive  stages  of  separation  are  necessary  to  get  material  that 
is  90  per  cent  or  more  U-235  or  deuterium.  Such  a  series  of 
successive  separating  stages  is  called  a  cascade  if  the  flow  is 
continuous  from  one  stage  to  the  next.  (A  fractionating  tower  of 
separate  plates  such  as  has  been  described  is  an  example  of  a 
simple  cascade  of  separating  units.)  A  complete  analysis  of  the 
problems  of  a  cascade  might  be  presented  in  general  terms. 
Actually  it  has  been  worked  out  by  R.  P.  Feynman  of  Princeton 
and  others  for  a  certain  type  of  electromagnetic  separator  and  by 
K.  Cohen  and  I.  Kaplan  of  Columbia,  by  M.  Benedict  and  A.  M. 
Squires  of  the  Kellex  Corporation  and  others  for  diffusion  proc- 
esses. At  present  we  shall  make  only  two  points  about  multiple- 
stage  or  "cascade"  plants. 

9.33.  The  first  point  is  that  there  must  be  recycling.  Con- 
sidering a  U-235  separation  plant,  the  material  fed  into  any  stage 

The  Separation  of  Isotopes  167 

above  the  first  has  already  been  enriched  in  U-235.  Part  of  this 
feed  material  may  be  further  enriched  in  passing  through  the 
stage  under  consideration.  The  remainder  will  typically  become 
impoverished  but  not  so  much  impoverished  as  to  be  valueless. 
It  must  be  returned  to  an  earlier  stage  and  recycled.  Even  the 
impoverished  material  from  the  first  (least  enriched)  stage  may  be 
worth  recycling;  some  of  the  U-235  it  still  contains  may  be  re- 
covered (stripped). 

9.34.  The  second  point  is  that  the  recycling  problem  changes 
greatly  at  the  higher  (more  enriched)  stages.  Assuming  steady 
stage  operation,  we  see  that  the  net  flow  of  uranium  through  the 
first  stage  must  be  at  least  140  times  as  great  as  through  the  last 
stage.  The  net  flow  in  any  given  stage  is  proportional  to  the  rela- 
tive concentration  of  U-238  and  thus  decreases  with  the  number  of 
stages  passed.  Since  any  given  sample  of  material  is  recycled  many 
times,  the  amount  of  material  processed  in  any  stage  is  far  greater 
than  the  net  flow  through  that  stage  but  is  proportional  to  it. 

9.35.  We  mention  these  points  to  illustrate  a  phase  of  the 
separation  problem  that  is  not  always  obvious,  namely,  that  the 
separation  process  which  is  best  for  an  early  stage  of  separation 
is  not  necessarily  best  for  a  later  stage.  Factors  such  as  those  we 
have  mentioned  differ  not  only  from  stage  to  stage  but  from 
process  to  process.  For  example,  recycling  is  far  simpler  in  a 
diffusion  plant  than  in  an  electromagnetic  plant.  A  plant  com- 
bining two  or  more  processes  may  well  be  the  best  to  accomplish 
the  overall  separation  required.  In  the  lower  (larger)  stages  the 
size  of  the  equipment  and  the  power  required  for  it  may  deter- 
mine the  choice  of  process.  In  the  higher  (smaller)  stages  these 
factors  are  outweighed  by  convenience  of  operation  and  hold-up 
time,  which  may  point  to  a  different  process. 


9.36.  The  next  two  chapters  are  devoted  to  descriptions  of  the 
three  methods  used  for  large-scale  separation  of  the  uranium 
isotopes.  These  are  the  only  isotope-separation  plants  that  have 

168  The  Separation  of  Isotopes 

turned  out  to  be  of  major  importance  to  the  project  up  to  the 
present  time.  At  an  earlier  stage  it  seemed  likely  that  the  centri- 
fuge might  be  the  best  method  for  separating  the  uranium  isotopes 
and  that  heavy  water  would  be  needed  as  a  moderator.  We  shall 
describe  briefly  the  centrifuge  pilot  plant  and  the  heavy  water 
production  plants. 


9.37.  Two  methods  were  used  for  the  concentration  of  deu- 
terium. These  were  the  fractional  distillation  of  water  and  the 
hydrogen-water  exchange  reaction  method. 

9.38.  The   first   of  these   follows   well   established   fractional 
distillation   methods   except   that  very  extensive  distillation   is 
required  because  of  the  slight  difference  in  boiling  point  of  light 
and  heavy  water.  Also,  because  of  this  same  small  difference,  the 
amount  of  steam  required  is  very  large.  The  method  is  very 
expensive  because  of  these  factors,  but  plants  could  be  con- 
structed with  a  minimum  of  development  work.  Plants  were 
started  by  du  Pont  in  January  1943,  and  were  put  into  operation 
about  January  1 944. 

9.39.  The  second  method  for  the  preparation  of  heavy  water 
depends   upon   the   catalytic  exchange   of  deuterium   between 
hydrogen  gas  and  water.  When  such  an  exchange  is  established 
by  catalysts,  the  concentration  of  the  deuterium  in  the  water  is 
greater  than  that  in  the  gas  by  a  factor  of  about  three  as  we  have 
already  seen. 

9.40.  In  this  process  water  is  fed  into  a  tower  and  flows  counter- 
currently  to  hydrogen  and  steam  in  an  intricate  manner.  At  the 
bottom  of  the  tower  the  water  is  converted  to  hydrogen  gas  and 
oxygen  gas  in  electrolytic  cells  and  the  hydrogen  is  fed  back  to 
the  bottom  of  the  tower  mixed  with  steam.  This  steam  and 
hydrogen  mixture  passes  through  beds  of  catalyst  and  bubbles 
through  the  downflowing  water.  Essentially,  part  of  the  deu- 
terium originally  in  the  hydrogen  concentrates  in  the  steam  and 
then  is  transferred  to  the  downflowing  water.  The  actual  plant 
consists  of  a  cascade  of  towers  with  the  largest  towers  at  the  feed 

The  Separation  of  Isotopes  169 

end  and  the  smallest  towers  at  the  production  end.  Such  a  cascade 
follows  the  same  general  principle  as  those  discussed  above  in 
connection  with  separation  problems  in  general.  This  process 
required  the  securing  of  very  active  catalysts  for  the  exchange 
reactions.  The  most  effective  catalyst  of  this  type  was  discovered 
by  H.  S.  Taylor  at  Princeton  University,  while  a  second,  less 
active  catalyst  was  discovered  by  A.  von  Grosse.  In  the  develop- 
ment of  these  catalysts  R.  H.  Crist  of  Columbia  University  made 
the  necessary  determinations  of  physical  constants  and  H.  R. 
Arnold  of  du  Pont  did  the  development  work  on  one  of  the 

9.41.  This  process  was  economical  in  operation.  The  plant  was 
placed  at  the  works  of  the  Consolidated  Mining  &  Smelting  Co., 
at  Trail,  British  Columbia,  Canada,  because  of  the  necessity  of 
using  electrolytic  hydrogen.  The  construction  of  the  plant  was 
under  the  direction  of  E.  V.  Murphree  and  F.  T.  Barr  of  the 
Standard  Oil  Development  Co. 


9.42.  For  a  long  time  in  the  early  days  of  the  project  the  gase- 
ous diffusion  method  and  the  centrifuge  method  were  considered 
the  two  separation  methods  most  likely  to  succeed  with  uranium. 
Both  were  going  to  be  difficult  to  realize  on  a  large  scale.  After 
the  reorganization  in  December  1941  research  and  development 
on  the  centrifuge  method  continued  at  the  University  of  Virginia 
and  at  the  Standard  Oil  Development  Company's  laboratory  at 
Bayway.  To  make  large  centrifuges  capable  of  running  at  very 
high  speeds  was  a  major  task  undertaken  by  the  Westinghouse 
Electric  and  Manufacturing  Company  of  East  Pittsburgh. 

9.43.  Because  of  the  magnitude  of  the  engineering  problems 
involved,  no  large-scale  production  plant  was  ever  authorized 
but  a  pilot  plant  was  authorized  and  constructed  at  Bayway. 
It  was  operated  successfully  and  gave  approximately  the  degree 
of  separation  predicted  by  theory.  This  plant  was  later  shut  down 
and  work  on  the  centrifuge  method  was  discontinued.  For  this 

170  The  Separation  of  Isotopes 

reason  no  further  discussion  of  the  centrifuge  method  is  given  in 
this  report. 


9.44.  The  most  important  methods  of  isotope  separation  that 
have  been  described  were  known  in  principle  and  had  been 
reduced  to  practice  before  the  separation  of  uranium  isotopes 
became  of  paramount  importance.  They  had  not  been  applied 
to  uranium  except  for  the  separation  of  a  few  micrograms,  and 
they  had  not  been  applied  to  any  substance  on  a  scale  comparable 
to  that  now  required.  But  the  fundamental  questions  were  of 
costs,  efficiency,  and  time,  not  of  principle;  in  other  words,  the 
problem  was  fundamentally  technical,  not  scientific.  The  plu- 
tonium  production  problem  did  not  reach  a  similar  stage  until 
after  the  first  self-sustaining  chain-reacting  pile  had  operated  and 
the  first  microgram  amounts  of  plutonium  had  been  separated. 
Even  after  this  stage  many  of  the  experiments  done  on  the 
plutonium  project  were  of  vital  interest  for  the  military  use  either 
of  U-235  or  plutonium  and  for  the  future  development  of  nuclear 
power.  As  a  consequence,  the  plutonium  project  has  continued 
to  have  a  more  general  interest  than  the  isotope  separation  proj- 
ects.  Many  special  problems  arose  in  the  separation  projects 
which  were  extremely  interesting  and  required  a  high  order  of 
scientific  ability  for  their  solution  but  which  must  still  be  kept 
secret.  It  is  for  such  reasons  that  the  present  non-technical  report 
has  given  first  emphasis  to  the  plutonium  project  and  will  give 
less  space  to  the  separation  projects.  This  is  not  to  say  that  the 
separation  problem  was  any  easier  to  solve  or  that  its  solution 
was  any  less  important. 


9.45.  Except  in  electromagnetic  separators,  isotope  separation 
depends  on  small  differences  in  the  average  behavior  of  molecules. 
Such  effects  are  used  in  six  "statistical"  separation  methods: 
(1)    gaseous   diffusion,    (2)   distillation,    (3)    centrifugation,    (4) 

The  Separation  of  Isotopes  171 

thermal  diffusion,  (5)  exchange  reactions,  (6)  electrolysis. 
Probably  only  (1),  (3),  and  (4)  are  suitable  for  uranium;  (2), 
(5),  and  (6)  are  preferred  for  the  separation  of  deuterium  from 
hydrogen.  In  all  these  "statistical"  methods  the  separation  factor 
is  small  so  that  many  stages  are  required,  but  in  the  case  of  each 
method  large  amounts  of  material  may  be  handled.  All  these 
methods  had  been  tried  with  some  success  before  1940;  however, 
none  had  been  used  on  a  large  scale  and  none  had  been  used  for 
uranium.  The  scale  of  production  by  electromagnetic  methods 
was  even  smaller  but  the  separation  factor  was  larger.  There 
were  apparent  limitations  of  scale  for  the  electromagnetic 
method.  There  were  presumed  to  be  advantages  in  combining 
two  or  more  methods  because  of  the  differences  in  performance 
at  different  stages  of  separation.  The  problem  of  developing  any 
or  all  of  these  separation  methods  was  not  a  scientific  one  of 
principle  but  a  technical  one  of  scale  and  cost.  These  develop- 
ments can  therefore  be  reported  more  briefly  than  those  of  the 
plutonium  project  although  they  are  no  less  important.  A  pilot 
plant  was  built  using  centrifuges  and  operated  successfully.  No 
large-scale  plant  was  built.  Plants  were  built  for  the  production 
of  heavy  water  by  two  different  methods. 





10.1.  It  was  in  February  1940  that  small  amounts  of  concen- 
trated fractions  of  the  three  uranium  isotopes  of  masses  234,  235, 
and  238  were  obtained  by  A.  O.  Nier  using  his  mass  spectrometer 
and  were  turned  over  to  E.  T.  Booth,  A.  von  Grosse,  and  J.  R. 
Dunning  for  investigation  with  the  Columbia  University  cyclo- 
tron. These  men  soon  demonstrated  that  U-235  was  the  isotope 
susceptible  to  fission  by  thermal  neutrons.  It  was  natural,  there- 
fore, that  this  group,  under  the  leadership  of  Dunning,  became 
more  interested  than  ever  in  the  large-scale  separation  of  the 
uranium  isotopes. 

10.2.  The    diffusion    method   was    apparently   first   seriously 
reviewed  by  Dunning  in  a  memorandum  to  G.  B.  Pegram,  which 
was  sent  to  L.  J.  Briggs  in  the  fall  of  1940.  This  memorandum 
summarized  preliminary  investigations  that  had  been  carried 
on  by  E.  T.  Booth,  A.  von  Grosse  and  J.  R.  Dunning.  Work  was 
accelerated  in  1941  with  financial  help  provided  by  a  contract 
that  H.  C.  Urey  had  recived  from  the  Navy  for  the  study  of 
isotope  separation — principally  by  the  centrifuge  method.  During 
this  period  F.  G.  Slack  of  Vanderbilt  University  and  W.  F. 
Libby  of  the  University  of  California  joined  the  group.  An  OSRD 
contract  (OEMsr-106)  calling  specifically  for  diffusion  studies 
went  into  effect  on  July  1,  1941,  and  ran  for  a  year.  The  work  con- 
tinued on  an  expanding  scale  under  a  series  of  OSRD  and  Army 
contracts  through  the  spring  of  1945.  Up  until  May  1943  Dun- 
ning was  in  immediate  charge  of  this  work;  Urey  was  in  charge 
of  statistical  methods  in  general.  From  that  time  until  February 
1945  Urey  was  in  direct  charge  of  the  Columbia  part  of  the 


Diffusion  Separation  173 

diffusion  work,  with  Dunning  continuing  as  director  of  one  of 
the  principal  divisions.  On  March  1,  1945,  the  laboratory  was 
taken  over  from  Columbia  by  Carbide  and  Carbon  Chemicals 
Corporation.  Early  in  1 942,  at  the  suggestion  of  E.  V.  Murphree, 
the  M.  W.  Kellogg  Company  was  brought  in  to  develop  plans 
for  large-scale  production  of  diffusion-plant  equipment  and 
eventually  to  build  a  full-scale  plant.  To  carry  out  this  under- 
taking, a  new  subsidiary  company  was  formed  called  the  Kellex 
Corporation.  In  January  1943,  Carbide  and  Carbon  Chemicals 
Corporation  was  given  the  responsibility  for  operating  the  plant. 

10.3.  As  stated  in  Chapter  IV,  by  the  end  of  1941  the  possi- 
bility of  separating  the  uranium  hexafluorides  had  been  demon- 
strated in  principle  by  means  of  a  single-stage  diffusion  unit 
employing  a  porous  barrier  (for  example,  a  barrier  made  by 
etching  a  thin  sheet  of  silver-zinc  alloy  with  hydrochloric  acid) . 
A  considerable  amount  of  work  on  barriers  and  pumps  had  also 
been  done  but  no  answer  entirely  satisfactory  for  large-scale 
operation  had  been  found.  Also,  K.  Cohen  had  begun  a  series 
of  theoretical  studies,  to  which  reference  has  already  been  made, 
as  to  what  might  be  the  best  way  to  use  the  diffusion  process, 
i.e.,  as  to  how  many  stages  would  be  required,  what  aggregate 
area  of  barrier  would  be  needed,  what  volume  of  gas  would  have 
to  be  circulated,  etc.  Theoretical  studies  and  process  develop- 
ment by  M.  Benedict  added  much  to  knowledge  in  this  field  and 
served  as  the  basis  of  design  of  the  large  plant. 

10.4.  Reports  received  from  the  British,  and  the  visit  by  the 
British  group  in  the  winter  of  1941-1942,  clarified  a  number  of 
points.  At  that  time  the  British  were  planning  a  diffusion  sepa- 
ration plant  themselves  so  that  the  discussions  with  F.  Simon,  R. 
Peierls,  and  others  were  particularly  valuable. 



10.5.  As  was  explained  in  the  last  chapter,  the  rate  of  diffusion 
of  a  gas  through  an  ideal  porous  barrier  is  inversely  proportional 

174  Diffusion  Separation 

to  the  square  root  of  its  molecular  weight.  Thus  if  a  gas  consisting 
of  two  isotopes  starts  to  diffuse  through  a  barrier  into  an  evacu- 
ated vessels,  the  lighter  isotope  (of  molecular  weight  MI)  diffuses 
more  rapidly  than  the  heavier  (of  molecular  weight  M2).  The 
result,  for  a  short  period  of  time,  at  least,  is  that  the  relative  con- 
centration of  the  lighter  isotope  is  greater  on  the  far  side  of  the 
barrier  than  on  the  near  side.  But  if  the  process  is  allowed  to 
continue  indefinitely,  equilibrium  will  become  established  and 
the  concentrations  will  become  identical  on  both  sides  of  the 
barrier.  Even  if  the  diffusate  gas  (the  gas  which  has  passed  through 
the  barrier)  is  drawn  away  by  a  pump,  the  relative  amount  of  the 
heavy  isotope  passing  through  the  barrier  will  increase  since 
the  light  isotope  on  the  near  side  of  the  barrier  has  been  depleted 
by  the  earlier  part  of  the  diffusion. 

10.6.  For  a  single  diffusion  operation,  the  increase  in  the  rela- 
tive concentration  of  the  light  isotope  in  the  diffused  gas  com- 
pared to  the  feed  gas  can  be  expressed  in  terms  of  the  separation 
factor  r  or  the  enrichment  factor,  r  —  1,  both  defined  in  para- 
graph 9.8  of  the  last  chapter.  A  rather  simple  equation  can  be 
derived  which  gives  r  —  1  in  terms  of  the  molecular  weights  and 
the  fraction  of  the  original  gas  which  has  diffused.  If  this  fraction 
is  very  small,  the  equation  reduces  to  r  =  a,  the  "ideal  separation 
factor"  of  paragraph  9.14.  If  the  fraction  diffused  is  appreciable, 
the  equation  shows  the  expected  diminution  in  separation.  For 
example,   if  half  the  gas  diffuses,  r  —  1  =  .69 (a  —  1),  or  for 
uranium  hexafluoride  r  =  1 .003  compared  to  the  value  of  1 .0043 
when  a  very  small  fraction  of  the  original  gas  has  diffused. 


10.7.  To   separate    the    uranium    isotopes,    many   successive 
diffusion  stages  (i.e.,  a  cascade)  must  be  used  since  a  —  1.0043 
for  U235F6  and  U238F6,  a  possible  gas  for  uranium  separation. 
Studies  by  Cohen  and  others  have  shown  that  the  best  flow 
arrangement  for  the  successive  stages  is  that  in  which  half  the 
gas  pumped  into  each  stage  diffuses  through  the  barrier,  the 
other  (impoverished)  half  being  returned  to  the  feed  of  the  next 

Diffusion  Separation  175 

lower  stage.  For  such  an  arrangement,  as  we  have  seen,  the  ideal 
separating  effect  between  the  feed  and  output  of  a  single  stage 
is  0.69(a  —  1).  This  is  often  called  e,  the  "overall  enrichment 
per  stage."  For  the  uranium  hexafluorides,  €  =  0.003,  in  theory; 
but  it  is  somewhat  less  in  practice  as  a  result  of  "back  diffusion," 
of  imperfect  mixing  on  the  high  pressure  side,  and  of  imperfec- 
tions in  the  barrier.  The  first  experimental  separation  of  the 
uranium  hexafluorides  (by  E.  T.  Booth,  H.  C.  Paxton,  and  C.  B. 
Slade)  gave  results  corresponding  to  €  =  0.0014.  If  one  desires 
to  produce  99  per  cent  pure  U235F6,  and  if  one  uses  a  cascade 
in  which  each  stage  has  a  reasonable  overall  enrichment  factor, 
then  it  turns  out  that  roughly  4,000  stages  are  required. 


10.8.  Of  the  gas  that  passes  through  the  barrier  of  any  given 
stage,  only  half  passes  through  the  barrier  of  the  next  higher 
stage,  the  other  half  being  returned  to  an  earlier  stage.  Thus 
most  of  the  material  that  eventually  emerges  from  the  cascade 
has  been  recycled  many  times.  Calculation  shows  that  for  an 
actual  uranium-separation  plant  it  may  be  necessary  to  force 
through  the  barriers  of  the  first  stage  100,000  times  the  volume 
of  gas  that  comes  out  the  top  of  the  cascade  (i.e.,  as  desired 
product  U235F6).  The  corresponding  figures  for  higher  stages 
fall  rapidly  because  of  reduction  in  amount  of  unwanted  material 
(U238F6)  that  is  carried  along. 



10.9.  By  the  time  of  the  general  reorganization  of  the  atomic- 
bomb  project  in  December  1 941 ,  the  theory  of  isotope  separation 
by  gaseous  diffusion  was  well  understood.  Consequently  it  was 
possible  to  define  the  technical  problems  that  would  be    en- 
countered in  building  a  large-scale  separation  plant.  The  decisions 
as  to  scale  and  location  of  such  plant  were  not  made  until  the 
\vinter  of  1942-1943,  that  is,  about  the  same  time  as  the  corre- 

176  Diffusion  Separation 

spending  decisions  were  being  made  for  the  plutonium  produc- 
tion plants. 


10.10.  The  general  objective  of  the  large-scale  gaseous  diffusion 
plant  was  the  production  each  day  of  a  specified  number  of 
grams  of  uranium  containing  of  the  order  of  ten  times  as  much 
U-235  as  is  present  in  the  same  quantity  of  natural  uranium. 
However,  it  was  apparent  that  the  plant  would  be  rather  flexible 
in  operation,  and  that  considerable  variations  might  be  made  in 
the  degree  of  enrichment  and  yield  of  the  final  product. 


10.11.  Uranium  hexafluoride  has  been  mentioned  as  a  gas 
that  might  be  suitable  for  use  in  the  plant  as  "process  gas";  not 
the  least  of  its  advantages  is  that  fluorine  has  only  one  isotope  so 
that  the  UFe  molecules  of  any  given  uranium  isotope  all  have  the 
same  mass.  This  gas  is  highly  reactive  and  is  actually  a  solid  at 
room  temperature  and  atmospheric  pressure.  Therefore  the  study 
of  other  gaseous  compounds  of  uranium  was  urgently  undertaken. 
As  insurance  against  failure  in  this  search  for  alternative  gases, 
it  was  necessary  to  continue  work  on  uranium  hexafluoride,  as 
in  devising  methods  for  producing  and  circulating  the  gas. 


10.12.  The  number  of  stages  required  in  the  main  cascade  of 
the  plant  depended  only  on  the  degree  of  enrichment  desired 
and  the  value  of  overall  enrichment  per  stage  attainable  with 
actual  barriers.  Estimates  were  made  which  called  for  several 
thousand  stages.  There  was  also  to  be  a  "stripping"  cascade  of 
several  hundred  stages,  the  exact  number  depending  on  how 
much  unseparated  U-235  could  economically  be  allowed  to  go 
to  waste. 


10.13.  We  have  seen  that  the  total  value  of  gas  that  must 
diffuse  through  the  barriers  is  very  large  compared  to  the  volume 

Diffusion  Separation  177 

of  the  final  product.  The  rate  at  which  the  gas  diffuses  through 
unit  area  of  barrier  depends  on  the  pressure  difference  on  the 
two  sides  of  the  barrier  and  on  the  porosity  of  the  barrier.  Even 
assuming  full  atmospheric  pressure  on  one  side  and  zero  pressure 
on  the  other  side,  and  using  an  optimistic  figure  for  the  porosity, 
calculations  showed  that  many  acres  of  barrier  would  be  needed 
in  the  large-scale  plant. 


10.14.  At  atmospheric  pressure  the  mean  free  path  of  a  mole- 
cule is  of  the  order  of  a  ten-thousandth  of  a  millimeter  or  one 
tenth  of  a  micron.  To  insure  true  "diffusive"  flow  of  the  gas,  the 
diameter  of  the  myriad  holes  in  the  barrier  must  be  less  than  one 
tenth  the  mean  free  path.  Therefore  the  barrier  material  must 
have  almost  no  holes  which  are  appreciably  larger  than  0.01 
micron  (4  X  10~7  inch),  but  must  have  billions  of  holes  of  this 
size  or  smaller.  These  holes  must  not  enlarge  or  plug  up  as  the 
result  of  direct  corrosion  or  dust  coming  from  corrosion  elsewhere 
in  the  system.  The  barrier  must  be  able  to  withstand  a  pressure 
"head"  of  one  atmosphere.  It  must  be  amenable  to  manufacture 
in  large  quantities  and  with  uniform  quality.  By  January  1 942,  a 
number  of  different  barriers  had  been  made  on  a  small  scale  and 
tested  for  separation  factor  and  porosity.  Some  were  thought  to 
be  very  promising,  but  none  had  been  adequately  tested  for  actual 
large-scale  production  and  plant  use. 


10.15.  In  any  given  stage  approximately  half  of  the  material 
entering  the  stage  passes  through  the  barrier  and  on  to  the  next 
higher  stage,  while  the  other  half  passes  back  to  the  next  lower 
stage.  The  diffused  half  is  at  low  pressure  and  must  be  pumped  to 
high  pressure  before  feeding  into  the  next  stage.  Even  the  undif- 
fused  portion  emerges  at  somewhat  lower  pressure  than  it  entered 
and  cannot  be  fed  back  to  the  lower  stage  without  pumping. 
Thus  the  total  quantity  of  gas  per  stage  (comprising  twice  the 

178  Diffusion  Separation 

amount  which  flows  through  the  barrier)  has  to  be  circulated  by 
means  of  pumps. 

10.16.  Since  the  flow  of  gas  through  a  stage  varies  greatly  with 
the  position  of  the  stage  in  the  cascade,  the  pumps  also  vary 
greatly  in  size  or  number  from  stage  to  stage.  The  type  and 
capacity  of  the  pump  required  for  a  given  stage  depends  not 
only  on  the  weight  of  gas  to  be  moved  but  on  the  pressure  rise 
required.  Calculations  made  at  this  time  assumed  a  fore  pressure 
of  one  atmosphere  and  a  back  pressure  (i.e.,  on  the  low  pressure 
side  of  the  barrier)  of  one  tenth  of  an  atmosphere.  It  was  esti- 
mated that  thousands   of  pumps  would   be  needed  and  that 
thousands  of  kilowatts  would  be  required  for  their  operation. 
Since  an  unavoidable  concomitant  of  pumping  gas  is  heating 
it,  it  was  evident  that  a  large  cooling  system  would  have  to  be 
provided.  By  early  1942,  a  good  deal  of  preliminary  work  had 
been  done  on  pumps.  Centrifugal  pumps  looked  attractive  in 
spite  of  the  problem  of  sealing  their  shafts,  but  further  experi- 
mental work  was  planned  on  completely  sealed  pumps  of  various 


10.17.  It  was  clear  that  the  whole  circulating  system  com- 
prising pumps,  barriers,  piping,  and  valves  would  have  to  be 
vacuum  tight.  If  any  lubricant  or  sealing  medium  is  needed  in 
the  pumps,  it  should  not  react  with  the  process  gas.  In  fact  none 
of  the  materials  in  the  system  should  react  with  the  process  gas 
since  such  corrosion  would  lead  not  only  to  plugging  of  the 
barriers  and  various  mechanical  failures  but  also  to  absorption 
(i.e.,  virtual  disappearance)  of  uranium  which  had  already  been 
partially  enriched. 


10.18.  In  an  ideal  cascade,  the  pumping  requirements  change 
from  stage  to  stage.  In  practice  it  is  not  economical  to  provide  a 
different  type  of  pump  for  every  stage.  It  is  necessary  to  deter- 
mine how  great  a  departure  from  the  ideal  cascade  (i.e.,  what 

Diffusion  Separation  179 

minimum  number  of  pump  types)  should  be  employed  in  the 
interest  of  economy  of  design,  repair,  etc.  Similar  compromises 
are  used  for  other  components  of  the  cascade. 


10.19.  When  first  started,  the  plant  must  be  allowed  to  run 
undisturbed  for  some  time,  until  enough  separation  has  been 
effected  so  that  each  stage  contains  gas  of  appropriate  enrich- 
ment. Only  after  such  stabilization  is  attained  is  it  desirable  to 
draw  off  from  the  top  stage  any  of  the  desired  product.  Both 
the  amount  of  material  involved  (the  hold-up)  and  the  time 
required  (the  start-up  time)  are  great  enough  to  constitute  major 
problems  in  their  own  right. 


10.20.  It  was  apparent  that  there  would  be  only  three  types  of 
material  loss  in  the  plant  contemplated,  namely:  loss  by  leakage, 
loss  by  corrosion  (i.e.,  chemical  combination  and  deposition), 
and  loss  in  plant  waste.  It  was  expected  that  leakage  could  be 
kept  very  small  and  that — after  an  initial  period  of  operation — 
loss  from  corrosion  would  be  small.  The  percentage  of  material 
lost  in  plant  waste  would  depend  on  the  number  of  stripping 


10.21.  Questions  as  to  how  the  barrier  material  was  to  be  used 
(whether  in  tubes  or  sheets,  in  large  units  or  small  units),  how 
mixing  was  to  be  effected,  and  what  controls  and  instruments 
would  be  required  were  still  to  be  decided.  There  was  little 
reason  to  expect  them  to  be  unanswerable,  but  there  was  no 
doubt  that  they  would  require  both  theoretical  and  experimental 


10.22.  By  1942  the  theory  of  isotope  separation  by  gaseous 
diffusion  had  been  well  worked  out,  and  it  became  clear  that  a 

180  Diffusion  Separation 

very  large  plant  would  be  required.  The  major  equipment  items 
in  this  plant  were  diffusion  barriers  and  pumps.  Neither  the 
barriers  nor  the  pumps  which  were  available  at  that  time  had 
been  proved  generally  adequate.  Therefore  the  further  develop- 
ment of  pumps  and  barriers  was  especially  urgent.  There  were 
also  other  technical  problems  to  be  solved,  these  involving  cor- 
rosion, vacuum  seals,  and  instrumentation. 


10.23.  As  we  mentioned  at  the  beginning  of  this  chapter,  the 
diffusion  work  was  initiated  by  J.  R.  Dunning.  The  work  was 
carried  on  under  OSRD  auspices  at  Columbia  University  until 
May  1,  1943,  when  it  was  taken  over  by  the  Manhattan  District. 
In  the  summer  of  1943  the  difficulties  encountered  in  solving 
certain  phases  of  the  project  led  to  a  considerable  expansion, 
particularly  of  the  chemical  group.  H.  C.  Urey,  then  director 
of  the  work,   appointed   H.   S.   Taylor  of  Princeton  associate 
director  and  added  E.  Mack,  Jr.  of  Ohio  State,  G.  M.  Murphy 
of  Yale,  and  P.  H.  Emmett  of  Johns  Hopkins  to  the  senior  staff. 
Most  of  the  work  was  moved  out  of  the  Columbia  laboratories 
to  a  large  building  situated  near  by.  The  chemists  at  Princeton 
who  had  been  engaged  in  heavy  water  studies  were  assigned  some 
of  the  barrier  research  problems.  Early  in  1944,  L.  M.  Currie 
of  the  National   Carbon   Company  became  another  associate 
director  to  help  Urey  in  his  liaison  and  administrative  work. 

10.24.  As  has  been  mentioned,  the  M.  W.  Kellogg  Company 
was  chosen  early  in  1942  to  plan  the  large  scale  plant.  For  this 
purpose  Kellogg  created  a  special  subsidiary  called  The  Kellex 
Corporation,  with  P.  C.  Leith  as  executive  in  charge  and  tech- 
nical head  and,  responsible  to  him,  A.  L.  Baker  as  Project  Man- 
ager, and  J.  H.  Arnold  as  Director  of  Research  and  Development. 
The  new  subsidiary  carried  on  research  and  development  in  its 
Jersey  City  laboratories  and  in  the  laboratory  building  referred 
to  in  the  paragraph  above;  developed  the  process  and  engineering 
designs;  and  procured  materials  for  the  large-scale  plant  and 
supervised  its  construction.  The  plant  was  constructed  by  the 

Diffusion  Separation  181 

J.  A.  Jones  Construction  Company,  Incorporated,  of  Charlotte, 
North  Carolina. 

10.24-a.  The  Kellex  Corporation,  unlike  conventional  indus- 
trial firms,  was  a  cooperative  of  scientists,  engineers  and  adminis- 
trators recruited  from  essentially  all  branches  of  industry  and 
gathered  for  the  express  purpose  of  carrying  forward  this  one  job. 
Service  was  on  a  voluntary  basis,  individuals  prominent  in  indus- 
try freely  relinquishing  their  normal  duties  and  responsibilities 
to  devote  full  time  to  Kellex  activities.  As  their  respective  tasks 
are  being  completed  these  men  are»  returning  to  their  former 
positions  in  industry. 

10.25.  In  January    1943,    Carbide   and    Carbon    Chemicals 
Corporation  were  chosen  to  be  the  operators  of  the  completed 
plant.  Their  engineers  soon  began  to  play  a  large  role  not  only 
in  the  planning  and  construction  but  also  in  the  research  work. 

PRODUCTION,  1942  TO  1945 


10.26.  Even  before  1942,  barriers  had  been  developed  that 
were   thought   to   be   satisfactory.    However,    the   barriers   first 
developed  by  E.  T.  Booth,  H.  C.  Paxton,  and  C.  B.  Slade  were 
never  used  on  a  large  scale  because  of  low  mechanical  strength 
and  poor  corrosion  resistance.  In  1942,  under  the  general  super- 
vision of  Booth  and  F.  G.  Slack  and  with  the  cooperation  of 
various  scientists  including  F.   C.  Nix  of  the  Bell  Telephone 
Laboratories,   barriers  of  a  different  type  were  produced.  At 
one  time,  a  barrier  developed  by  E.  O.  Norris  and  E.  Adler 
was  thought  sufficiently  satisfactory  to  be  specified  for  plant  use. 
Other    barriers    were    developed    by   combining   the    ideas   of 
several  men  at  the  Columbia  laboratories  (by  now  christened  the 
SAM  Laboratories),  Kellex,  Bell  Telephone  Laboratories,  Bake- 
lite  Corporation,   Houdaille-Hershey  Corporation,   and  others. 
The  first  specimens  of  the  type  of  barrier  selected  for  general  use 
in  the  plant  were  prepared  by  C.  A.  Johnson  of  Kellex,  and  the 

182  Diffusion  Separation 

barrier  was  perfected  under  the  general  supervision  of  H.  S. 
Taylor.  One  modification  of  this  barrier  developed  by  the  SAM 
Laboratories  represented  a  marked  improvement  in  quality  and 
is  being  used  in  a  large  number  of  stages  of  the  plant.  By  1945  the 
problem  was  no  longer  one  of  barely  meeting  minimum  specifica- 
tions, but  of  making  improvements  resulting  in  greater  rate  of 
output  or  greater  economy  of  operation. 

10.27.  Altogether  the  history  of  barrier  development  reminds 
the  writer  of  the  history  of  the  "canning"  problem  of  the  plu- 
tonium  project.  In  each  case  the  methods  were  largely  cut  and 
try,  and  satisfactory  or  nearly  satisfactory  solutions  were  repeat- 
edly announced;  but  in  each  case  a  really  satisfactory  solution 
was  not  found  until  the  last  minute  and  then  proved  to  be  far 
better  than  had  been  hoped. 


10.28.  The  early  work  on  pumps  was  largely  under  the  super- 
vision of  H.  A.  Boorse  of  Columbia  University.  When  Kellex  came 
into  the  picture  in  1942,  its  engineers,  notably  G.  W.  Watts, 
J.  S.  Swearingen  and  O.  C.  Brewster,  took  leading  positions  in 
the  development  of  pumps  and  seals.  It  must  be  remembered 
that  these  pumps  are  to  be  operated  under  reduced  pressure,  must 
not  leak,  must  not  corrode,  and  must  have  as  small  a  volume  as 
possible.  Many  different  types  of  centrifugal  blower  pumps  and 
reciprocating  pumps  were  tried.  In  one  of  the  pumps  for  the 
larger  stages,  the  impeller  is  driven  through  a  coupling  contain- 
ing a  very  novel  and  ingenious  type  of  seal.  Another  type  of  pump 
is  completely  enclosed,  its  centrifugal  impeller  and  rotor  being 
run  from  outside,  by  induction. 


10.29.  As  in  the  plutonium  problem,  so  here  also,  there  were 
many  questions  of  corrosion,  etc.,  to  be  investigated.  New  coolants 
and  lubricants  were  developed  by  A.  L.  Henne  and  his  associ- 
ates, by  G.  H.  Cady,  by  WT.  T.  Miller  and  his  co-workers,  by 
E.  T.  McBee  and  his  associates,   and  by  scientists  of  various 

Diffusion  Separation  183 

corporations  including  Hooker  Electrochemical  Co.,  the  du  Pont 
Co.  and  the  Harshaw  Chemical  Co.  The  research  and  develop- 
ment and  plant  requirements  for  these  materials  and  other  special 
chemicals  were  coordinated  by  R.  Rosen,  first  under  OSRD  and 
later  for  Kellex.  Methods  of  pretreating  surfaces  against  corrosion 
were  worked  out.  Among  the  various  instruments  designed  or 
adapted  for  project  use,  the  mass  spectrograph  deserves  special 
mention.  The  project  was  fortunate  in  having  the  assistance  of 
A.  O.  Nier  of  the  University  of  Minnesota  and  later  of  Kellex 
whose  mass  spectrograph  methods  of  isotope  analysis  were  suffi- 
ciently advanced  to  become  of  great  value  to  the  project,  as  in 
analyzing  samples  of  enriched  uranium.  Mass  spectrographs 
were  also  used  in  pretesting  parts  for  vacuum  leaks  and  for 
detecting  impurities  in  the  process  gas  in  the  plant. 


10.30.  Strictly  speaking,  there  was  no  pilot  plant.  That  is  to 
say,  there  was  no  small-scale  separation  system  set  up  using  the 
identical  types  of  blowers,  barriers,  barrier  mountings,  cooling, 
etc.,  that  were  put  into  the  main  plant.  Such  a  system  could 
not  be  set  up  because  the  various  elements  of  the  plant  were  not 
all  available  prior  to  the  construction  of  the  plant  itself.  To  pro- 
ceed with  the  construction  of  the  full-scale  plant  under  these 
circumstances  required  foresight  and  boldness. 

10.31.  There  was,  however,  a  whole  series  of  so-called  pilot 
plants  which  served  to  test  various  components  or  groups  of 
components  of  the  final  plant.  Pilot  plant  No.  1  was  a  12-stage 
plant  using  a  type  of  barrier  rather  like  that  used  in  the  large- 
scale  plant,  but  the  barrier  material  was  not  fabricated  in  the 
form  specified  for  the  plant  and  the  pumps  used  were  sylphon- 
sealed  reciprocating  pumps,  not  centrifugal  pumps.  Work  on  this 
plant  in  1943  tested  not  only  the  barriers  and  general  system  of 
separation  but  gave  information  about  control  valves,  pressure 
gauges,  piping,  etc.  Pilot  plant  No.  2,  a  larger  edition  of  No.  1 
but  with  only  six  stages,  was  used  in  late  1943  and  early  1944, 
particularly  as  a  testing  unit  for  instruments.  Pilot  plant  No.  3a, 

184  Diffusion  Separation 

using  centrifugal  blowers  and  dummy  diffusers,  was  also  intended 
chiefly  for  testing  instruments.  Pilot  plant  No.  3b  was  a  real 
pilot  plant  for  one  particular  section  of  the  large-scale  plant.  Pilot 
plants  using  full-scale  equipment  at  the  plant  site  demonstrated 
the  vacuum  tightness,  corrosion  resistance  and  general  operability 
of  the  equipment. 


10.32.  In  December  1942,  the  Kellogg  Company  was  author- 
ized to  proceed  with  preliminary  plant  design  and  in  January 
1943  the  construction  of  a  plant  was  authorized. 


10.33.  As  stated  in  an  earlier  chapter,  a  site  in  the  Tennessee 
Valley  had  originally  been  chosen  for  all  the  Manhattan  District 
plants,  but  the  plutonium  plant  was  actually  constructed  else- 
where.   There    remained    the    plutonium    pilot    plant    already 
described,    the    gaseous    diffusion    plant,    the    electromagnetic 
separation  plant  (see  Chapter  XI),  and  later  the  thermal  dif- 
fusion plant  which  were  all  built  in  the  Tennessee  Valley  at  the 
Clinton  site,  known  officially  as  the  Clinton  Engineer  Works. 

•  10.34.  This  site  was  examined  by  Colonel  Marshall,  Colonel 
Nichols,  and  representatives  of  Stone  and  Webster  Engineering 
Corporation  in  July  1942,  and  its  acquisition  was  recommended. 
This  recommendation  was  endorsed  by  the  OSRD  S-l  Executive 
Committee  at  a  meeting  in  July  1942.  Final  approval  was  given 
by  Major  General  L.  R.  Groves  after  personal  inspection  of  the 
70-square-mile  site.  In  September  1942,  the  first  steps  were  taken 
to  acquire  the  tract,  which  is  on  the  Clinch  River  about  thirty 
miles  from  Knoxville,  Tennessee,  and  eventually  considerably 
exceeded  70  square  miles.  The  plutonium  pilot  plant  is  located 
in  one  valley,  the  electromagnetic  separation  plant  in  an  adjoin- 
ing one,  and  the  diffusion  separation  plant  in  a  third. 

10.35.  Although  the  plant  and  site  development  at  Hanford 
is  very  impressive,  it  is  all  under  one  company  dealing  with  but 
one  general  operation  so  that  it  is  in  some  respects  less  interesting 

Diffusion  Separation  185 

than  Clinton,  which  has  a  great  multiplicity  of  activity.  To 
describe  the  Clinton  site,  with  its  great  array  of  new  plants,  its 
new  residential  districts,  new  theatres,  new  school  system,  seas 
of  mud,  clouds  of  dust,  and  general  turmoil  is  outside  the  scope 
of  this  report. 


10.36.  Construction  of  the  steam  power  plant  for  the  diffusion 
plant  began  on  June  1,  1943.  It  is  one  of  the  largest  such  power 
plants  ever  built.   Construction  of  other  major  buildings  and 
plants  started  between  August  29,  1943  and  September  10,  1943. 


10.37.  Unlike  Hanford,  the  diffusion  plant  consists  of  so  many 
more  or  less  independent  units  that  it  was  put  into  operation 
section  by  section,  as  permitted  by  progress  in  constructing  and 
testing.  Thus  there  was  no  dramatic  start-up  date  nor  any  unto- 
ward incident  to  mark  it.  The  plant  was  in  successful  operation 
before  the  summer  of  1945. 

10.38.  For  the  men  working  on  gaseous  diffusion  it  was  a  long 
pull  from  1940  to  1945,  not  lightened  by  such  exciting  half-way 
marks  as  the  first  chain-reacting  pile  at  Chicago.  Perhaps  more 
than  any  other  group  in  the  project,  those  who  have  worked  on 
gaseous  diffusion  deserve  credit  for  courage  and  persistence  as 
well  as  scientific  and  technical  ability.  For  security  reasons,  we 
have  not  been  able  to  tell  how  they  solved  their  problems — even 
in   many   cases   found   several   solutions,   as   insurance   against 
failure  in  the  plant.  It  has  been  a  notable  achievement.  In  these 
five  years  there  have  been  periods  of  discouragement  and  pessi- 
mism. They  are  largely  forgotten  now  that  the  plant  is  not  only 
operating  but  operating  consistently,  reliably,  and  with  a  per- 
formance better  than  had  been  anticipated. 


10.39.  Work  at   Columbia   University  on  the  separation  of 
isotopes  by  gaseous  diffusion  began  in  1940,  and  by  the  end  of 

186  Diffusion  Separation 

1942  the  problems  of  large-scale  separation  of  uranium  by  this 
method  had  been  well  defined.  Since  the  amount  of  separation 
that  could  be  effected  by  a  single  stage  was  very  small,  several 
thousand  successive  stages  were  required.  It  was  found  that 
the  best  method  of  connecting  the  many  stages  required  extensive 
recycling  so  that  thousands  of  times  as  much  material  would  pass 
through  the  barriers  of  the  lower  stages  as  would  ultimately 
appear  as  product  from  the  highest  stage. 

10.40.  The  principal  problems  were  the  development  of  satis- 
factory barriers  and  pumps.  Acres  of  barrier  and  thousands  of 
pumps  were  required.  The  obvious  process  gas  was  uranium  hexa- 
fluoride  for  which  the  production  and  handling  difficulties  were 
so  great  that  a  search  for  an  alternative  was  undertaken.  Since 
much  of  the  separation  was  to  be  carried  out  at  low  pressure, 
problems    of  vacuum    technique    arose,    and    on   a   previously 
unheard-of  scale.  Many  problems  of  instrumentation  and  control 
were  solved;  extensive  use  was  made  of  various  forms  of  mass 

10.41.  The  research  was  carried  out  principally  at  Columbia 
under  Dunning  and  Urey.  In  1942,  the  M.  W.  Kellogg  Company 
was  chosen  to  develop  the  process  and  equipment  and  to  design 
the  plant  and  set  up  the  Kellex  Corporation  for  the  purpose.  The 
plant  was  built  by  the  J.  A.  Jones  Construction  Company.  The 
Carbide   and   Carbon   Chemicals   Corporation  was  selected  as 
operating  company. 

10.42.  A  very  satisfactory  barrier  was  developed  although  the 
final  choice  of  barrier  type  was  not  made  until  the  construction 
of  the  plant  was  well  under  way  at  Clinton  Engineer  Works  in 
Tennessee.  Two  types  of  centrifugal  blower  were  developed  to 
the  point  where  they  could  take  care  of  the  pumping  require- 
ments. The  plant  was  put  into  successful  operation  before  the 
summer  of  1945. 





11.1.  In  Chapter  IV  we  said  that  the  possibility  of  large-scale 
separation  of  the  uranium  isotopes  by  electromagnetic  means  was 
suggested  in  the  fall  of  1941  by  E.  O.  Lawrence  of  the  University 
of  California  and  H.  D.  Smyth  of  Princeton  University.  In  Chap- 
ter IX  we  described  the  principles  of  one  method  of  electromag- 
netic separation  and  listed  the  three  limitations  of  that  method: 
difficulty  of  producing  ions,  limited  fraction  of  ions  actually  used, 
and  space  charge  effects. 

11.2.  By  the  end  of  December  1941,  when  the  reorganization 
of  the  whole  uranium  project  was  effected,  Lawrence  had  already 
obtained  some  samples  of  separated  isotopes  of  uranium  and  in 
the  reorganization  he  was  officially  placed  in  charge  of  the  prepa- 
ration of  further  samples  and  the  making  of  various  associated 
physical    measurements.    However,   just    as    the    Metallurgical 
Laboratory  very  soon  shifted  its  objective  from  the  physics  of 
the  chain  reaction  to  the  large-scale  production  of  plutonium, 
the  objective  of  Lawrence's  division  immediately  shifted  to  the 
effecting  of  large-scale  separation  of  uranium  isotopes  by  electro- 
magnetic methods.  This  change  was  prompted  by  the  success 
of  the  initial  experiments  at  California  and  by  the  development 
at  California  and  at  Princeton  of  ideas  on  other  possible  methods. 
Of  the  many  electromagnetic  schemes  suggested,  three  soon  were 
recognized  as  being  the  most  promising:  the  "calutron"  mass 
separator,   the  magnetron-type  separator  later  developed  into 
the  "ionic  centrifuge,"  and  the  "isotron"  method  of  ''bunching" 
a  beam  of  ions.  The  first  two  of  these  approaches  were  followed 
at  California  and  the  third  at  Princeton.  After  the  first  few 


188  Electromagnetic  Separation 

months,  by  far  the  greatest  effort  was  put  on  the  calutron,  but 
some  work  on  the  ionic  centrifuge  was  continued  at  California 
during  the  summer  of  1942  and  was  further  continued  by  J. 
Slepian  at  the  Westinghouse  laboratories  in  Pittsburgh  on  a 
small  scale  through  the  winter  of  1944-1945.  Work  on  the  isotron 
was  continued  at  Princeton  until  February  1943,  when  most  of 
the  group  was  transferred  to  other  work.  Most  of  this  chapter 
will  be  devoted  to  the  calutron  since  that  is  the  method  that  has 
resulted  in  large-scale  production  of  U-235.  A  brief  description 
will  also  be  given  of  the  thermal  diffusion  plant  built  to  provide 
enriched  feed  material  for  the  electromagnetic  plant. 

11.3.  Security  requirements  make  it  impossible  here — as  for 
other  parts  of  the  project — to  present  many  of  the  most  interesting 
technical  details.  The  importance  of  the  development  is  con- 
siderably greater  than  is  indicated  by  the  amount  of  space  which 
is  given  it  here. 



11.4.  A.  O.  Nier's  mass  spectrograph  was  set  up  primarily  to 
measure  relative  abundances  of  isotopes,  not  to  separate  large 
samples.  Using  vapor  from  uranium  bromide  Nier  had  prepared 
several  small  samples  of  separated  isotopes  of  uranium,  but  his 
rate  of  production  was  very  low  indeed,  since  his  ion  current 
amounted  to  less  than  one  micro-ampere.  (A  mass  spectrograph 
in   which  one   micro-ampere   of  normal   uranium   ions   passes 
through  the  separating  fields  to  the  collectors  will  collect  about 
one  microgram  of  U-235  per  16-hour  day.)  The  great  need  of 
samples  of  enriched  U-235  for  nuclear  study  was  recognized 
early  by  Lawrence,  who  decided  to  see  what  could  be  done  with 
the  help  of  the  37-inch  (cyclotron)  magnet  at  Berkeley.  The 
initial  stages  of  this  work  were  assisted  by  a  grant  from  the  Re- 
search Corporation  of  New  York,  which  was  later  repaid.  Be- 
ginning January  1,  1942,  the  entire  support  came  from  the  OSRD 
through  the  S-l    Committee.   Later,   as  in  other  parts  of  the 

Electromagnetic  Separation  189 

uranium  project,  the  contracts  were  taken  over  by  the  Manhattan 

11.5.  At  Berkeley,  after  some  weeks  of  planning,  the  37-inch 
cyclotron  was  dismantled  on  November  24,  1941,  and  its  magnet 
was  used  to  produce  the  magnetic  field  required  in  what  came 
to  be  called  a  "calutron"  (a  name  representing  a  contraction  of 
"California  University  cyclotron").  An  ion  source  consisting  of 
an  electron  beam  traversing  the  vapor  of  a  uranium  salt  was  set 
up  corresponding  to  the  ion  source  shown  in  the  drawing  in 
Chap.  IX,  p.  164.  Ions  were  then  accelerated  to  the  slit  82  through 
which  they  passed  into  the  separating  region  where  the  magnetic 
field  bent  their  paths  into  semicircles  terminating  at  the  collector 
slit.  By  December  1,  1941,  molecular  ion  beams  from  the  residual 
gas  were  obtained,  and  shortly  thereafter  the  beam  consisting 
of  singly  charged  uranium  ions  (U+)  was  brought  up  to  an 
appreciable  strength.  It  was  found  that  a  considerable  proportion 
of  the  ions  leaving  the  source  were  U+  ions.  For  the  purpose  of 
testing  the  collection  of  separated  samples,  a  collector  with  two 
pockets  was  installed,   the  two  pockets  being  separated  by  a 
distance  appropriate  to  the  mass  numbers  235  and  238.  Two 
small  collection  runs  using  U+  beams  of  low  strength  were  made 
in  December,  but  subsequent  analyses  of  the  samples  showed 
only  a  small  separation  factor.  By  the  middle  of  January  1942, 
a  run  had  been  made  with  a  reasonable  beam  strength  and  an 
aggregate  flow  or  through-put  of  appreciable  amount  which 
showed  a  much  improved  separation  factor.  By  early  February 
1 942,  beams  of  much  greater  strength  were  obtained,  and  Law- 
rence reported  that  good  separation  factors  were  obtainable  with 
such  beams.  By  early  March  1942,  the  ion  current  had  been 
raised  still  further.  These  results  tended  to  bear  out  Lawrence's 
hopes  that  space  charge  could  be  neutralized  by  ionization  of  the 
residual  gas  in  the  magnet  chamber. 


11.6.  By  this  time  it  was  clear  that  the  calutron  was  potentially 
able  to  effect  much  larger  scale  separations  than  had  ever  before 

190  Electromagnetic  Separation 

been  approached  by  an  electromagnetic  method.  It  was  evi- 
dently desirable  to  explore  the  whole  field  of  electromagnetic 
separation.  With  this  end  in  view,  Lawrence  mobilized  his  group 
at  the  Radiation  Laboratory  of  the  University  of  California  at 
Berkeley  and  began  to  call  in  others  to  help.  Among  those 
initially  at  Berkeley  were  D.  Cooksey,  P.  C.  Aebersold,  W.  M. 
Brobeck,  F.  A.  Jenkins,  K.  R.  MacKenzie,  W.  B.  Reynolds, 
D.  H.  Sloan,  F.  Oppenheimer,  J.  G.  Backus,  B.  Peters,  A.  C. 
Helmholz,  T.  Finkelstein,  and  W.  E.  Parkins,  Jr.  Lawrence 
called  back  some  of  his  former  students,  including  R.  L.  Thornton, 
J.  R.  Richardson,  and  others.  Among  those  working  at  Berkeley 
for  various  periods  were  L.  P.  Smith  from  Cornell,  E.  U.  Condon 
and  J.  Slepian  from  Westinghouse,  and  I.  Langmuir  and 
K.  H.  Kingdon  from  General  Electric.  During  this  early  period 
J.  R.  Oppenheimer  was  still  at  Berkeley  and  contributed  some 
important  ideas.  In  the  fall  of  1 943  the  group  was  further  strength- 
ened by  the  arrival  of  a  number  of  English  physicists  under  the 
leadership  of  M.  L.  Oliphant  of  the  University  of  Birmingham. 

11.7.  Initially  a  large  number  of  different  methods  were  con- 
sidered and  many  exploratory  experiments  were  performed.  The 
main  effort,  however,  soon  became  directed  towards  the  develop- 
ment of  the  calutron,  the  objective  being  a  high  separation  factor 
and  a  large  current  in  the  positive  ion  beam. 


11.8.  Of  the   three   apparent   limitations  listed   in   the   first 
paragraph — difficulty  of  producing  ions,  limited  fraction  of  ions 
actually  used,  and  space  charge  effects — only  the  last  had  yielded 
to  the  preliminary  attack.  Apparently  space  charge  in  the  neigh- 
borhood of  the  positive  ion  beam  could  be  nullified  to  a  very 
great  extent.  There  remained  as  the  immediate  objectives  a  more 
productive  ion  source  and  more  complete  utilization  of  the  ions. 

11.9.  The  factors  that  control  the  effectiveness  of  an  ion  source 
are  many.  Both  the  design  of  the  source  proper  and  the  method 
of  drawing  ions  from  it  are  involved.  The  problems  to  be  solved 
cannot  be  formulated  simply  and  must  be  attacked  by  methods 

Electromagnetic  Separation  191 

that  are  largely  empirical.  Even  if  security  restrictions  permitted 
an  exposition  of  the  innumerable  forms  of  ion  source  and  acceler- 
ating system  that  were  tried,  such  exposition  would  be  too 
technical  to  present  here. 

11.10.  Turning  to  the  problem  of  effecting  more  complete 
utilization  of  the  ions,  we  must  consider  in  some  detail  the  princi- 
ple of  operation  of  the  calutron.  The  calutron  depends  on  the 
fact  that  singly  charged  ions  moving  in  a  uniform  magnetic 
field  perpendicular  to  their  direction  of  motion  are  bent  into 
circular  paths  of  radius  proportional  to  their  momenta.  Con- 
sidering now  just  a  single  isotope,  it  is  apparent  that  the  ions 

passing  through  the  two  slits  (and  thus  passing  into  the  large 
evacuated  region  in  which  the  magnetic  field  is  present)  do  not 
initially  follow  a  single  direction,  but  have  many  initial  directions 
lying  within  a  small  angle,  whose  size  depends  on  the  width  of 
the  slits.  Fortunately,  however,  since  all  the  ions  of  the  isotope 
in  question  follow  curved  paths  of  the  same  diameter,  ions  starting 
out  in  slightly  different  directions  tend  to  meet  again — or  almost 
meet  again — after  completing  a  semicircle.  It  is,  of  course,  at 
this  position  of  reconvergence  that  the  collector  is  placed. 
Naturally,  the  ions  of  another  isotope  (for  example,  ions  of  mass 
238  instead  of  235)  behave  similarly,  except  that  they  follow 
circles  of  slightly  different  diameter.  Samples  of  the  two  isotopes 
are  caught  in  collectors  at  the  two  different  positions  of  recon- 
vergence. Now  the  utilization  of  a  greater  fraction  of  the  ions 
originally  produced  may  be  accomplished  readily  enough  by 
widening  the  two  slits  referred  to.  But  to  widen  the  slits  to  any 
great  extent  without  sacrificing  sharpness  of  focus  at  the  recon- 

192  Electromagnetic  Separation 

vergence  positions  is  not  easy.  Indeed  it  can  be  accomplished 
only  by  use  of  carefully  proportioned  space  variations  in  the 
magnetic  field  strength.  Fortunately,  such  variations  were  worked 
out  successfully. 

11.11.  Another  problem,  not  so  immediate  but  nevertheless 
recognized  as  important  to  any  production  plant,  was  that  of 
more  efficient  use  of  the  magnetic  field.  Since  large  electro- 
magnets are  expensive  both  to  build  and  to  operate,  it  was 
natural  to  consider  using  the  same  magnetic  field  for  several  ion 
beams.   The  experimental  realization  of  such  an  economical 
scheme  became  a  major  task  of  the  laboratory. 


11.12.  Although  the  scale  of  separation  reached  by  March  1 942 
was  much  greater  than  anything  that  had  previously  been  done 
with  an  electromagnetic  mass  separator,  it  was  still  very  far  from 
that  required  to  produce  amounts  of  material  that  would  be  of 
military  significance.  The  problems  that  have  been  outlined  not 
only  had  to  be  solved,  but  they  had  to  be  solved  on  a  grand  scale. 
The  37-inch  cyclotron  magnet  that  had  been  used  was  still 
capable  of  furnishing  useful  information,  but  larger  equipment 
was  desirable.  Fortunately  a  very  much  larger  magnet,  intended 
for  a  giant  cyclotron,  had  been  under  construction  at  Berkeley. 
This  magnet,  with  a  pole  diameter  of  184  inches  and  a  pole  gap 
of  72  inches,  was  to  be  the  largest  in  existence.  Work  on  it  had 
been  interrupted  because  of  the  war,  but  it  was  already  suffi- 
ciently advanced  so  that  it  could  be  finished  within  a  few  months 
if  adequate  priorities  were  granted.  Aside  from  the  magnet  itself, 
the  associated  building,  laboratories,  shops,  etc.,  were  almost 
ideal  for  the  development  of  the  calutron.  Needless  to  say,  work 
was  resumed  on  the  giant  magnet  and  by  the  end  of  May  1942, 
it  was  ready  for  use.  * 

*  The  construction  of  the  giant  cyclotron  had  been  undertaken  with 
private  funds  largely  supplied  by  the  Rockefeller  Foundation,  augmented 
by  donations  from  the  Research  Corporation,  the  John  and  Mary  Markel 
Foundation,  and  the  University  of  California.  In  order  to  push  the  construe- 

Electromagnetic  Separation  193 


11.13.  The  first  experiments  using  the  37-inch  magnet  have 
been  described  in  a  previous  paragraph.   Later  developments 
proceeded  principally  along  these  two  lines:  construction  and 
installation  of  a  properly  engineered  separation   unit  for  the 
37-inch  magnet,  and  design  and  construction  of  experimental 
separation  units  to  go  into  the  big  magnet. 

11.14.  Besides  the  gradual  increase  in  ion  beam  strength  and 
separation  factor  that  resulted  from  a  series  of  developments  in 
the  ion  source  and  in  the  accelerating  system,  the  hoped-for 
improvement  in  utilization  of  ions  was  achieved  during  the 
summer  of  1 942,  using  the  giant  magnet.  Further,  it  was  possible 
to  maintain  more  than  one  ion  beam  in  the  same  magnetic 
separating  region.  Experiments  on  this  latter  problem  did  run 
into  some  difficulties,  however,  and  it  appeared  that  there  might 
be  limitations  on  the  number  of  sources  and  receivers  that  could 
be  put  in  a  single  unit  as  well  as  on  the  current  that  could  be 
used  hi  each  beam  without  spoiling  the  separation. 

11.15.  It  was  evident  that  many  separator  units  would  be 
needed  to  get  an  amount  of  production  of  military  significance. 
Therefore,  consideration  was  given  to  various  systems  of  com- 
bining groups  of  units  in  economical  arrangements.  A  scheme 
was  worked  out  which  was  later  used  in  the  production  plants 
and  which  has  proved  satisfactory. 


11.16.  In  September  1942,  both  the  gaseous  diffusion  and  the 
centrifugal  methods  of  uranium  isotope  separation  had  been 
under  intensive  study — and  for  a  longer  period  than  in  the  case 
of  the  electromagnetic 'method.  Both  of  these  methods — gaseous 
diffusion   and   centrifuge — looked   feasible   for  large-scale   pro- 
duction of  U-235,  but  both  would  require  hundreds  of  stages  to 
achieve  large-scale  separation.  Neither  had  actually  produced 

tion  as  fast  as  possible  overtime  work  was  required  at  additional  expense. 
To  cover  these  costs  the  Rockefeller  Foundation  made  an  extra  appropriation. 

194  Electromagnetic  Separation 

any  appreciable  amounts  of  separated  U-235.  No  large-scale 
plant  for  plutonium  production  was  under  way,  and  the  self- 
sustaining  chain  reaction  which  was  to  produce  plutonium  had 
not  yet  been  proved  attainable.  But  in  the  case  of  the  electro- 
magnetic method,  after  the  successful  separation  of  milligram 
amounts,  there  was  no  question  as  to  the  scientific  feasibility. 
If  one  unit  could  separate  10  mg  a  day,  100,000,000  units  could 
separate  one  ton  a  day.  The  questions  were  of  cost  and  time. 
Each  unit  was  to  be  a  complicated  electromagnetic  device  re- 
quiring high  vacuum,  high  voltages,  and  intense  magnetic  fields; 
and  a  great  deal  of  research  and  development  work  would  be 
required  before  complete,  large-scale,  units  could  be  constructed. 
Many  skilled  operators  would  probably  be  needed.  Altogether,  at 
that  time  it  looked  very  expensive,  but  it  also  looked  certain  and 
relatively  quick.  Moreover,  the  smallness  of  the  units  had  the 
advantage  that  development  could  continue,  modifications  could 
be  made  in  the  course  of  construction  or,  within  limits,  after 
construction,  and  Capacity  could  always  be  expanded  by  building 
new  units. 


11.17.  On  the  basis  of  rather  incomplete  scientific  and  engi- 
neering information  on  all  the  methods  and  on  the  basis  of 
equally  dubious  cost  estimates,  decisions  had  to  be  made  on  three 
issues:  (1)  whether  to  build  an  electromagnetic  plant;  (2)  how 
big  such  a  plant  should  be;  (3)  at  what  point  of  development  the 
design  should  be  frozen. 


11.18.  On  the  strength  of  the  results  reported  on  experiments 
at  Berkeley  in  the  summer  of  1942,  the  S-l  Executive  Committee, 
at  a  meeting  at  Berkeley  on  September  1 3-14,  1 942,  recommended 
that  commitments  be  made  by  the  Army  for  an  electromagnetic 
separation  plant  to  be  built  at  the  Tennessee  Valley  site  (Clinton 
Engineer  Works).  It  was  recommended  that  it  should  be  agreed 
that  commitments  for  this  plant  might  be  cancelled  on  the  basis 

Electromagnetic  Separation  195 

of  later  information.  It  was  recommended  that  a  pilot  plant 
should  be  erected  at  the  Tennessee  Valley  site  as  soon  as  possible. 
(However,  this  recommendation  was  subsequently  withdrawn  and 
such  a  pilot  plant  was  never  built.)  The  construction  of  a  pro- 
duction plant  was  authorized  by  General  Groves  on  November  5, 
1942,  with  the  understanding  that  the  design  for  the  first  units 
was  to  be  frozen  immediately. 


11.19.  In  describing  the  production  of  plutonium,  we  discussed 
the  division  of  responsibility  between  the  Metallurgical  Project 
and  the  du  Pont  Company.  The  electromagnetic  separation  plant 
was  planned  and  built  under  a  somewhat  different  scheme  of 
organization.  The  responsibility  was  divided  between  six  major 
groups.  The  Radiation  Laboratory  at  the  University  of  California 
was  responsible  for  research  and  development;  the  Westinghouse 
Electric  and  Manufacturing  Company  for  making  the  mechanical 
parts,  i.e.,  sources,  receivers,  pumps,  tanks,  etc.;  the  General 
Electric  Company  for  the  electrical  equipment  and  controls; 
the  Allis-Chalmers  Company  for  the  magnets;  the  Stone  and 
Webster  Engineering  Company  for  the  construction  and  assembly; 
and  the  Tennessee  Eastman  Company  for  operation.  All  five 
industrial  concerns  kept  groups  of  their  engineers  at  Berkeley 
so  that  a  system  of  frequent  informal  conference  and  cross- 
checking was  achieved.  Thus  the  major  part  of  the  planning  was 
done  cooperatively  in  a  single  group,  even  though  the  details 
might  be  left  to  the  home  offices  of  the  various  companies. 


11.20.  Strangely   enough,    although   the   theory   of  the   self- 
sustaining  chain-reacting  pile  is  already  well  worked  out,  the 
theory  of  gaseous  discharge,  after  fifty  years  of  intensive  study, 
is  still  inadequate  for  the  prediction  of  the  exact  behavior  of  the 
ions  in  a  calutron.  The  amount  of  U-235  collected  per  day,  and 
the  purity  of  the  material  collected,  are  affected  by  many  factors, 
including:   (1)  the  width,  spacing,  and  shape  of  the  collector, 

196  Electromagnetic  Separation 

(2)  the  pressure  in  the  .magnet  space,  (3)  the  strength  and  uni- 
formity of  the  magnetic  field,  (4)  the  shape  and  spacing  of  the 
defining  slits  and  accelerating  system,  (5)  the  accelerating  voltage, 
(6)  the  size  and  shape  of  the  slit  in  the  arc  source  from  which 
the  ions  come,  (7)  the  current  in  the  arc,  (8)  the  position  of  the 
arc  within  the  arc  chamber,  (9)  the  pressure  of  vapor  in  the  arc 
chamber,  (10)  the  chemical  nature  of  the  vapor.  Evidently  there 
was  not  time  for  a  systematic  study  of  all  possible  combinations  of 
variables.  The  development  had  to  be  largely  intuitive.  A  variety 
of  conditions  had  to  be  studied  and  a  number  of  partial  interpre- 
tations had  to  be  made.  Then  the  accumulated  experience  of  the 
group,  the  "feel"  of  the  problem,  had  to  be  translated  into  specific 
plans  and  recommendations. 


11.21.  (a)   The  Number  of  Stages.  As  in  all  methods,  a  compro- 
mise must  be  made  between  yield  and  separation  factor.  In  the 
electromagnetic  system,  the  separation  factor  is  much  higher 
than  in  other  systems  so  that  the  number  of  stages  required  is 
small.  There  was  a  possibility  that  a  single  stage  might  be  suffi- 
cient. Early  studies  indicated  that  attempts  to  push  the  separation 
factor  so  high  as  to  make  single-stage  operation  feasible  cut  the 
yield  to  an  impractically  small  figure. 

11.22.  (b)  Specifications.  The  information  and  experience  that 
had  been  acquired  on  the  variables  such  as  those  mentioned 
above  had  to  be  translated  into  decisions  on  the  following  princi- 
pal points  before  design  could  actually  begin:  (1)  the  size  of  a 
unit  as  determined  by  the  radius  of  curvature  of  the  ion  path, 
the  length  of  the  source  slit,  and  the  arrangement  of  sources 
and  receivers;  (2)  the  maximum  intensity  of  magnetic  field  re- 
quired; (3)  whether  or  not  to  use  large  divergence  of  ion  beams; 
(4)  the  number  of  ion  sources  and  receivers  per  unit;  (5)  whether 
the  source  should  be  at  high  potential  or  at  ground  potential; 
(6)  the  number  of  accelerating  electrodes  and  the  maximum 
potentials  to  be  applied  to  them;  (7)  the  power  requirements  for 
arcs,  accelerating  voltages,  pumps,  etc.;  (8)  pumping  require- 

Electromagnetic  Separation  197 

ments;  (9)  number  of  units  per  pole  gap;  (10)  number  of  units 
per  building. 


11.23.  Most  of  the  design  features  for  the  first  plant  had  to  be 
frozen  in  the  fall  of  1942  on  the  basis  of  results  obtained  with 
runs  made  using  the  giant  magnet  at  Berkeley.  The  plant  design, 
however,  called  for  units  of  a  somewhat  different  type.  While 
there  was  no  reason  to  suppose  that  these  changes  would  introduce 
any  difference  in  performance,  it  was  obviously  desirable  to  build 
a  prototype  unit  at  Berkeley.  The  construction  of  this  unit  was 
approved  at  about  the  same  time  that  the  first  plant  units  were 
ordered  so  that  experience  with  it  had  no  influence  on  funda- 
mental design,  but  it  was  finished  and  operating  by  April  1943, 
that  is,  six  months  before  the  first  plant  unit.  Consequently,  it 
was  invaluable  for  testing  and  training  purposes.  Later,  a  third 
magnet  was  built  in  the  big  magnet  building  at  Berkeley.  All 
told,  there  have  been  six  separator  units  available  simultaneously 
for  experimental  or  pilot  plant  purposes  at  Berkeley.   Much 
auxiliary  work  has  also  been  done  outside  the  complete  units. 


11.24.  As  we  have  already  said,  H.  D.  Smyth  of  Princeton 
became  interested  in  electromagnetic  methods  of  separation  in 
the  late  summer  and  fall  of  1941.  He  was  particularly  interested 
in  devising  some  method  of  using  an  extended  ion  source  and 
beam  instead  of  one  limited  essentially  to  one  dimension  by  a 
slit  system  as  in  the  calutron  mass  separator.  A  method  of  actually 
achieving  separation  using  an  extended  ion  source  was  suggested 
by  R.  R.  Wilson  of  Princeton.  The  device  which  resulted  from 
Wilson's  ideas  was  given   the  deliberately  meaningless  name 

11.25.  The  isotron  is  an  electromagnetic  mass  separator  using 
an  extended  source  of  ions,  in  contrast  to  the  slit  sources  used  in 
ordinary  mass  spectrographs.  The  ions  from  the  extended  source 
are  first  accelerated  by  a  constant,  high-intensity,  electric  field 

198  Electromagnetic  Separation 

and  are  then  further  accelerated  by  a  low-intensity  electric  field 
varying  at  radio  frequency  and  in  "saw  tooth"  manner.  The 
effect  of  the  constant  electric  field  is  to  project  a  strong  beam  of 
ions  down  a  tube  with  uniform  kinetic  energy  and  therefore 
with  velocities  inversely  proportional  to  the  square  root  of  the 
masses  of  ions.  The  varying  electric  field,  on  the  other  hand, 
introduces  small,  periodic  variations  in  ion  velocity,  and  has  the 
effect  of  causing  the  ions  to  "bunch"  at  a  certain  distance  down 
the  tube.  (This  same  principle  is  used  in  the  klystron  high- 
frequency  oscillator,  where  the  electrons  are  "bunched"  or 
"velocity-modulated.")  The  bunches  of  ions  of  different  mass 
travel  with  different  velocities  and  therefore  become  separated. 
At  the  position  (actually  on  area  perpendicular  to  the  beam) 
where  this  occurs,  an  analyzer  applies  a  transverse  focussing 
electric  field  with  a  radio  frequency  component  synchronized 
with  the  arrival  of  the  bunches.  The  synchronization  is  such  that 
the  varying  component  of  the  transverse  field  strength  is  zero 
when  the  U-235  ion  bunches  come  through  and  a  maximum 
when  the  U-238  ion  bunches  come  through.  The  U-235  beams 
are  focussed  on  a  collector,  but  the  U-238  bunches  are  deflected. 
Thus  the  separation  is  accomplished. 

11.26.  This  scheme  was  described  at  the  December  18,  1941 
meeting  of  the  Uranium  Committee  and  immediately  thereafter 
was  discussed  more  fully  with  Lawrence,  who  paid  a  visit  to 
Princeton.  The  promise  of  the  method  seemed  sufficient  to  justify 
experimental  work,  which  was  begun  immediately  under  an 
OSRD  contract  and  continued  until  February  1943.  Since  the 
idea  involved  was  a  novel  one,  there  were  two  outstanding  issues: 
(1)  whether  the  method  would  work  at  all;  (2)  whether  it  could 
be  developed  for  large-scale   production   promptly  enough   to 
compete    with    the    more    orthodox    methods    already    under 

11.27.  An  experimental  isotron  was  constructed  and  put  into 
operation  by  the  end  of  January  1942.  Preliminary  experiments 
at  that  time  indicated  that  the  isotopes  of  lithium  could  be 
separated    by    the    method.    The    first   successful    collection    of 

Electromagnetic  Separation  199 

partially  separated  uranium  isotopes  was  made  in  the  spring 
of  1942. 

11.28.  Unfortunately,  progress  during  the  summer  and  fall  of 
1942  was  not  as  rapid  as  had  been  hoped.  Consequently,  it  was 
decided  to  close  down  the  Princeton  project  in  order  to  permit 
sending  the  personnel  to  the  site  where  the  atomic-bomb  labora- 
tory was  about  to  get  under  way.  Before  the  group  left  Princeton 
a  small  experimental  isotron  collected  several  samples  of  partly 
separated  uranium.  Thus,  the  method  worked;  but  its  large-scale 
applicability  was  not  fully  investigated. 


11.29.  In  December  1941,  when  the  whole  subject  of  isotope 
separation  was  under  discussion  at  Berkeley,  the  magnetron  was 
suggested  as  a  possible  mass  separator.  In  the  meantime,  Smyth 
of  Princeton  had  been  in  contact  with  L.  P.  Smith  of  Cornell 
and  had  discovered  that  Smith  and  his  students  had  done  a 
considerable  amount  of  work — and  with  evidence  of  success — 
on  the  separation  of  the  isotopes  of  lithium  by  just  such  a  method. 
This  was  reported  to  Lawrence  in  Washington  at  one  of  the 
December,  1941,  meetings  of  the  Uranium  Committee.  Lawrence 
immediately  got  in  touch  with  Smith,  with  the  result  that  Smith 
worked  on   the   method   at  Berkeley  from  February   1942   to 
June  1942.  J.  Slepian  of  the  Westinghouse  Research  Laboratory 
in  East  Pittsburgh  came  to  Berkeley  in  the  winter  of  1941-1942 
at  Lawrence's  invitation  and  became  interested  in  a  modification 
of  the  magnetron  which  he  called  an  ionic  centrifuge.  Slepian 
stayed  at  Berkeley  most  of  the  time  until  the  fall  of  1942,  after 
which  he  returned  to  East  Pittsburgh  where  he  continued  the 

11.30.  No  separation  of  uranium  was  actually  attempted  in 
the  magnetron.  Experiments  with  lithium  with  low  ion  currents 
showed  some  separation,  but  no  consistent  results  were  obtained 
with  high  ion  currents.  In  the  case  of  the  ionic  centrifuge,  uranium 
samples  have  been  collected  showing  appreciable  separation,  but 
the  results  have  not  been  clear-cut  or  consistent. 

200  Electromagnetic  Separation 


11.31.  With  the  virtual  elimination  of  the  isotron  and  the 
ionic  centrifuge  from  the  development  program,  the  calutron 
separator  became  the  only  electromagnetic  method  worked  on 
intensively.  Construction  of  initial  units  of  a  plant  had  been 
authorized  and  designs  had  been  frozen  for  such  units,  but  the 
whole  electromagnetic  program  had  been  in  existence  for  only 
a  little  more  than  a  year  and  it  was  obvious  that  available 
designs  were  based  on  shrewd  guesses  rather  than  on  adequate 
research.  A  similar  situation  might  have  occurred  with  the  chain- 
reacting  pile  if  unlimited  amounts  of  uranium  and  graphite  had 
been  available  before  the  theory  had  been  worked  out  or  before 
the  nuclear  constants  had  been  well  determined.  Fortunately  the 
nature  of  the  two  projects  was  very  different,  making  it  a  less 
speculative   venture   to   build   an   electromagnetic   plant   unit 
hastily  than  would  have  been  the  case  for  the  pile.  Further  re- 
search and  development  could   proceed  advantageously  even 
while  initial  units  of  the  plant  were  being  built  and  operated. 

TO  JUNE  1945 


11.32.  The  preceding  chapters  show  that  the  end  of  1942  was 
a  time  of  decision  throughout  the  uranium  project.  For  it  was  at 
that  time  that  a  self-sustaining  chain  reaction  was  first  produced, 
that  construction  was  authorized  for  the  Hanford  plutonium 
plant,  the  diffusion  plant  at  Clinton,  and  the  electromagnetic 
plant  at  Clinton.  The  diffusion  plant  was  more  flexible  than  the 
plutonium  plant,  since  the  diffusion  plant  could  be  broken  down 
into  sections  and  stages,  built  in  whole  or  in  part,  to  produce 
varying  amounts  of  U-235  of  varying  degrees  of  enrichment. 
The  electromagnetic  plant  was  even  more  flexible,  since  each 
separator  unit  was  practically  independent  of  the  other  units. 
The  separation  process  consisted  of  loading  a  charge  into  a  unit, 

Electromagnetic  Separation  201 

running  the  unit  for  a  while,  then  stopping  it  and  removing  the 
product.  To  be  sure,  the  units  were  built  in  groups,  but  most  of 
the  controls  were  separate  for  each  unit.  This  feature  made  it 
possible  to  build  the  plant  in  steps  and  to  start  operating  the  first 
part  even  before  the  second  was  begun.  It  was  also  possible  to 
change  the  design  of  subsequent  units  as  construction  proceeded; 
within  limits  it  was  possible  even  to  replace  obsolescent  units  in 
the  early  groups  with  new  improved  units. 


11.33.  Construction  of  the  first  series  of  electromagnetic  units 
at  Clinton  began  in  March  of  1943  and  this  part  of  the  plant  was 
ready  for  operation  in  November  1943.  The  group  at  Berkeley 
continued  to  improve  the  ion  sources,  the  receivers,  and  the 
auxiliary  equipment,  aiming  always  at  greater  ion  currents.  In 
fact,  Berkeley  reports  describe  no  less  than  seventy-one  different 
types  of  source  and  one  hundred  and  fifteen  different  types  of 
receiver,  all  of  which  reached  the  design  stage  and  most  of  which 
were  constructed  and  tested.  As  soon  as  the  value  of  a  given  design 
change  was  proved,  every  effort  was  made  to  incorporate  it  in 
the  designs  of  new  units. 

11.34.  Such  developments  as  these  required  constant  inter- 
change   of    information    among  laboratory,   engineering,   con- 
struction,  and  operating  groups.   Fortunately  the  liaison   was 
excellent.  The  companies  stationed  representatives  at  Berkeley, 
and  members  of  the  research  group  at  Berkeley  paid  frequent 
and  prolonged  visits  to  the  plant  at  Clinton.  In  fact,  some  of  the 
research  men  were  transferred  to  the  payroll  of  the  Tennessee 
Eastman  Company  operating  the  plant  at  Clinton,  and  a  group 
of  over  one  hundred  physicists  and  research  engineers  still  kept 
on  the  Berkeley  payroll  were  assigned  to  Clinton.  Particularly 
in  the  early  stages  of  operation  the  Berkeley  men  stationed  at 
Clinton  were  invaluable  as  "trouble  shooters"  and  hi  instructing 
operators.  A  section  of  the  plant  continued  to  be  maintained  as 
a  pilot  unit  for  testing  modified  equipment  and  revised  operating 
procedures,  and  was  run  jointly  by  the  Berkeley  group  and  by 

202  Electromagnetic  Separation 

Tennessee  Eastman.  In  addition  to  the  British  group  under 
Oliphant  already  mentioned,  there  was  a  British  group  of  chemists 
at  Clinton  under  J.  W.  Baxter. 


11.35.  Originally,  the  uranium  salts  used  as  sources  of  vapor 
for  the  ion-producing  arcs  had  not  been  investigated  with  any 
very  great  thoroughness  at  Berkeley,  but  as  the  process  developed, 
a  good  deal  of  work  was  done  on  these  salts,  and  a  search  was 
made  for  a  uranium  compound  that  would  be  better  than  that 
originally  used.  Some  valuable  studies  were  also  made  on  methods 
of  producing  the  compound  chosen. 

11.36.  By  far  the  most  important  chemical  problem  was  the 
recovery  of  the  processed  uranium  compounds  from  the  separa- 
tion units.  This  recovery  problem  had  two  phases.  In  units  of 
the  first  stage  it  was  essential  to  recover  the  separated  uranium 
from  the  receivers  with  maximum  efficiency;  whereas  recovery 
of  the  scattered  unseparated  uranium  from  other  parts  of  the  unit 
was  less  important.  But  if  higher  stage  units  are  used  even  the 
starting  material  contains  a  high  concentration  of  U-235,  and  it 
is  essential  to  recover  all  the  material  in  the  unit  at  the  end  of 
each  run,  i.e.,  material  remaining  in  the  ion  source  and  material 
deposited  on  the  accelerating  electrodes,  on  the  walls  of  the 
magnet  chamber,  and  on  the  receiver  walls. 


11.37.  For  nearly  a  year  the  electromagnetic  plant  was  the 
only  one  in  operation.  Therefore  the  urge  to  increase  its  produc- 
tion rate  was  tremendous.  It  was  realized  that  any  method  of 
enriching — even  slightly  enriching — the  material  to  be  fed  into 
the  plant  would  increase  the  production  rate  appreciably.  For 
example,  an  electromagnetic  unit  that  could  produce  a  gram  a 
day  of  40  per  cent  pure  U-235  from  natural  uranium  could 
produce  two  grams  a  day  of  80  per  cent  U-235  if  the  concentra- 
tion of  U-235  in  the  feed  material  was  twice  the  natural  concen- 
tration (1.4  per  cent  instead  of  0.7  per  cent). 

Electromagnetic  Separation  203 

11.38.  We  have  already  referred  to  the  work  done  by  P.  H. 
Abelson  of  the  Naval  Research  Laboratory  on  the  separation  of 
the  uranium  isotopes  by  thermal  diffusion  in  a  liquid  compound 
of  uranium.  By  the  spring  of  1943  Abelson  had  set  up  a  pilot 
plant  that  accomplished  appreciable  separation  of  a  considerable 
quantity  of  uranium  compound.  It  was  therefore  proposed  that 
a   large-scale   thermal   diffusion   plant   should   be   constructed. 
Such  a  plant  would  be  cheaper  than  any  of  the  other  large-scale 
plants,  and  it  could  be  built  more  quickly.  Its  principal  drawback 
\\  as  its  enormous  consumption  of  steam,  which  made  it  appear 
impracticable  for  the  whole  job  of  separation. 

11.39.  Not  only  was  a  pilot  plant  already  in  operation  at  the 
Naval   Research  Laboratory,   but  a  second,   somewhat  larger 
plant  was  under  construction  at  the  Philadelphia  Navy  Yard. 
Through  the  cooperation  of  the  Navy  both  the  services  of  Abelson 
and  the  plans  for  a  large-scale  plant  were  made  available  to  the 
Manhattan  District.   It  was  decided   to  erect  the  large-scale 
thermal  diffusion  plant  at  Clinton  (using  steam  from  the  power 
plant  constructed  for  the  gaseous  diffusion  plant)  and  to  use  the 
thermal-diffusion-plant  product  as  feed  material  for  the  electro- 
magnetic plant. 

11.40.  This  new  thermal  diffusion  plant  was  erected  in  amaz- 
ingly short  time  during  the  late  summer  of  1944.  In  spite  of  some 
disappointments,  operation  of  this  plant  has  succeeded  in  its 
purpose  of  considerably  increasing  the  production  rate  of  the 
electromagnetic  plant.  It  has  also  stimulated  work  on  the  uranium 
recovery  problem.  The  future  of  this  plant  is  uncertain.  Oper- 
ation of  the  gaseous-diffusion  plant  makes  it  difficult  to  get 
enough  steam  to  operate  the  thermal  diffusion  plant,  but  also 
furnishes  another  user  for  its  product. 


11.41.  Although  the  scientific  and  technical  problems  which 
confronted  the  Berkeley  groups  were  probably  not  as  varied  or 
numerous  as  the  problems  encountered  at  Chicago  and  Columbia, 
they  were  nevertheless  numerous.  Thus  many  problems  arose  in 

204  Electromagnetic  Separation 

the  designing  of  the  electric  power  and  control  circuits,  magnetic 
fields,  insulators,  vacuum  pumps,  tanks,  collectors,  and  sources. 
Many  equipment  items  had  to  be  designed  from  scratch  and  then 
mass-produced  under  high  priority. 


11.42.  The  electromagnetic  separation  plant  was  in  large-scale 
operation  during  the  winter  of  1944-1945,  and  produced  U-235 
of  sufficient  purity  for  use  in  atomic  bombs.  Its  operating  efficiency 
is   being   continually   improved.   Research  work  is   continuing 
although  on  a  reduced  scale. 


11.43.  In   the  early  days  of  the  uranium   project,   electro- 
magnetic methods  of  isotope  separation  were  rejected  primarily 
because  of  the  expected  effects  of  space-charge.  In  the  fall  of 
1941  the  question  was  reopened;  experiments  at  Berkeley  showed 
that   space-charge  effects  could   be   largely  overcome.    Conse- 
quently a  large-scale  program  for  the  development  of  electro- 
magnetic methods  was  undertaken. 

11.44.  Of  the  various  types  of  electromagnetic  methods  pro- 
posed, the  calutron  (developed  at  Berkeley)  received  principal 
attention.  Two  other  novel  methods  were  studied,  one  at  Berkeley 
and  one  at  Princeton.  The  calutron  mass  separator  consists  of  an 
ion  source  from  which  a  beam  of  uranium  ions  is  drawn  by  an 
electric   field,   an   accelerating  system   in   which   the   ions   are 
accelerated  to  high  velocities,  a  magnetic  field  in  which  the  ions 
travel  in  semicircles  of  radius  depending  on  ion  mass,  and  a 
receiving  system.  The  principal  problems  of  this  method  involved 
the  ion  source,  accelerating  system,  divergence  of  the  ion  beam, 
space  charge,  and  utilization  of  the  magnetic  field.  The  chief 
advantages  of  the  calutron  were  large  separation  factor,  small 
hold-up,  short  start-up  time,  and  flexibility  of  operation.  By  the 
fall  of  1942  sufficient  progress  had  been  made  to  justify  authoriza- 
tion of  plant  construction,  and  a  year  later  the  first  plant  units 
were  ready  for  trial  at  the  Clinton  Engineer  Works  in  Tennessee. 

Electromagnetic  Separation  205 

11.45.  Research  and  development  work  on  the  calutron  were 
carried  out  principally  at  the  Radiation  Laboratory  of  the  Uni- 
versity of  California,  under  the  direction  of  Lawrence.  Westing- 
house,    General    Electric,    and    Allis    Chalmers    constructed    a 
majority  of  the  parts;  Stone  and  Webster  built  the  plant,  and 
Tennessee  Eastman  operated  it. 

11.46.  Since  the  calutron  separation  method  was  one  of  batch 
operations  in  a  large  number  of  largely  independent  units,  it  was 
possible  to  introduce  important  improvements  even  after  plant 
operation  had  begun. 

11.47.  In  the  summer  of  1944  a  thermal-diffusion  separation 
plant  was  built  at  the  Clinton  Engineer  Works  to  furnish  en- 
riched feed  material  for  the  electromagnetic  plant  and  thereby 
increase  the  production  rate  of  this  latter  plant.  The  design  of  the 
thermal-diffusion  plant  was  based  on  the  results  of  research  car- 
ried out  at  the  Naval  Research  Laboratory  and  on  the  pilot  plant 
built  by  the  Navy  Department  at  the  Philadelphia  Navy  Yard. 

11.48.  Although  research  work  on  the  calutron  was  started 
later  than  on  the  centrifuge  and  diffusion  systems,  the  calutron 
plant  was  the  first  to  produce  large  amounts  of  the  separated 
isotopes  of  uranium. 



12.1.  The  entire  purpose  of  the  work  described  in  the  preceding 
chapters  was  to  explore  the  possibility  of  creating  atomic  bombs 
and  to  produce  the  concentrated  fissionable  materials  which 
would  be  required  in  such  bombs.  In  the  present  chapter,  the  last 
stage  of  the  work  will  be  described — the  development  at  Los 
Alamos  of  the  atomic  bomb  itself.  As  in  other  parts  of  the  project, 
there  are  two  phases  to  be  considered:  the  organization,  and  the 
scientific  and  technical  work  itself.  The  organization  will  be 
described  briefly;  the  remainder  of  the  chapter  will  be  devoted 
to  the  scientific  and  technical  problems.  Security  considerations 
prevent  a  discussion  of  many  of  the  most  important  phases  of  this 


12.2.  The  project  reorganization  that  occurred  at  the  begin- 
ning of  1942,  and  the  subsequent  gradual  transfer  of  the  work 
from   OSRD   auspices  to   the   Manhattan   District  have   been 
described  in  Chapter  V.  It  will  be  recalled  that  the  responsibili- 
ties   of   the    Metallurgical    Laboratory    at    Chicago    originally 
included  a  preliminary  study  of  the  physics  of  the  atomic  bomb. 
Some  such    studies  were  made  in   1941;   and  early  in   1942 
G.  Breit  got  various  laboratories  (see  Chapter  VI,  paragraph 
6.38)  started  on  the  experimental  study  of  problems  that  had  to 
be  solved  before  progress  could  be  made  on  bomb  design.  As  has 
been  mentioned  in  Chapter  VI,  J.  R.  Oppenheimer  of  the  Uni- 
versity of  California  gathered  a  group  together  in  the  summer  of 
1942  for  further  theoretical  investigation  and  also  undertook  to 
coordinate  this  experimental  work.  This  group  was  officially 


Work  on  the  Atomic  Bomb  207 

under  the  Metallurgical  Laboratory  but  the  theoretical  group 
did  most  of  its  work  at  the  University  of  California.  By  the  end 
of  the  summer  of  1 942,  when  General  L.  R.  Groves  took  charge 
of  the  entire  project,  it  was  decided  to  expand  the  work  consider- 
ably, and,  at  the  earliest  possible  time,  to  set  up  a  separate 

12.3.  In  the  choice  of  a  site  for  this  atomic-bomb  laboratory, 
the  all-important  considerations  were  secrecy  and  safety.  It  was 
therefore  decided  to  establish  the  laboratory  in  an  isolated  loca- 
tion and  to  sever  unnecessary  connection  with  the  outside  world. 

12.4.  By  November   1942   a  site  had  been  chosen — at  Los 
Alamos,  New  Mexico.  It  was  located  on  a  mesa  about  30  miles 
from  Santa  Fe.  One  asset  of  this  site  was  the  availability  of  con- 
siderable area  for  proving  grounds,  but  initially  the  only  struc- 
tures on  the  site  consisted  of  a  handful  of  buildings  which  once 
constituted  a  small  boarding  school.  There  was  no  laboratory, 
no  library,  no  shop,  no  adequate  power  plant.  The  sole  means  of 
approach  was  a  winding  mountain  road.  That  the  handicaps  of 
the  site  were  overcome  to  a  considerable  degree  is  a  tribute  to 
the  unstinting  efforts  of  the  scientific  and  military  personnel. 

12.5.  J.  R.  Oppenheimer  has  been  director  of  the  laboratory 
from  the  start.  He  arrived  at  the  site  in  March  1943,  and  was 
soon  joined  by  groups  and  individuals  from  Princeton  Univer- 
sity, University  of  Chicago,  University  of  California,  University 
of  Wisconsin,  University  of  Minnesota,  and  elsewhere.  With  the 
vigorous  support  of  General  L.  R.  Groves,  J.  B.  Conant,  and 
others,  Oppenheimer  continued  to  gather  around  him  scientists 
of  recognized  ability,  so  that  the  end  of  1 944  found  an  extraordi- 
nary galaxy  of  scientific  stars  gathered  on  this  New  Mexican 
mesa.  The  recruiting  of  junior  scientific  personnel  and  technicians 
was  more  difficult,  since  for  such  persons  the  disadvantages  of  the 
site  were  not  always  counterbalanced  by  an  appreciation  of  the 
magnitude  of  the  goal;  the  use  of  Special  Engineer  Detachment 
personnel  improved  the  situation  considerably. 

12.6.  Naturally,  the  task  of  assembling  the  necessary  apparatus, 
machines,  and  equipment  was  an  enormous  one.  Three  carloads 

208  Work  on  the  Atomic  Bomb 

of  apparatus  from  the  Princeton  project  filled  some  of  the  most 
urgent  requirements.  A  cyclotron  from  Harvard,  two  Van  de 
Graaff  generators  from  Wisconsin,  and  a  Cockcroft-Walton 
high- voltage  device  from  Illinois  soon  arrived.  As  an  illustration 
of  the  speed  with  which  the  laboratory  was  set  up,  we  may  record 
that  the  bottom  pole  piece  of  the  cyclotron  magnet  was  not  laid 
until  April  14,  1943,  yet  the  first  experiment  was  performed  in 
early  July.  Other  apparatus  was  acquired  in  quantity;  subsidiary 
laboratories  were  built.  Today  this  is  probably  the  best-equipped 
physics  research  laboratory  in  the  world. 

12.7.  The  laboratory  was  financed  under  a  contract  between 
the  Manhattan  District  and  the  University  of  California. 



12.8.  In  Chapter  II  we  stated  the  general  conditions  required 
to  produce  a  self-sustaining  chain  reaction.  It  was  pointed  out 
that  there  are  four  processes  competing  for  neutrons:   (1)  the 
capture  of  neutrons  by  uranium  which  results  in  fission;    (2) 
non-fission   capture   by   uranium;    (3)    non-fission   capture    by 
impurities;  and  (4)  escape  of  neutrons  from  the  system.  Therefore 
the  condition  for  obtaining  such  a  chain  reaction  is  that  process 
(1)  shall  produce  as  many  new  neutrons  as  are  consumed  or  lost 
in  all  four  of  the  processes.  It  was  pointed  out  that  (2)  may  be 
reduced  by  removal  of  U-238  or  by  the  use  of  a  lattice  and 
moderator,  that  (3)  may  be  reduced  by  achieving  a  high  degree 
of  chemical  purity,  and  that  (4)  may  be  reduced  (relatively)  by 
increasing  the  size  of  the  system.  In  our  earlier  discussions  of 
chain  reactions  it  was  always  taken  for  granted  that  the  chain- 
reacting  system  must  not  blow  up.  Now  we  want  to  consider  how 
to  make  it  blow  up. 

12.9.  By  definition,  an  explosion  is  a  sudden  and  violent  release 
of  a  large  amount  of  energy  in  a  small  region.  To  produce  an 
efficient  explosion  in  an  atomic  bomb,  the  parts  of  the  bomb 
must  not  become   appreciably  separated   before   a  substantial 

Work  on  the  Atomic  Bomb  209 

fraction  of  the  available  nuclear  energy  has  been  released,  since 
expansion  leads  to  increased  escape  of  neutrons  from  the  system 
and  thus  to  premature  termination  of  the  chain  reaction.  Stated 
differently,  the  efficiency  of  the  atomic  bomb  will  depend  on  the 
ratio  of  (a)  the  speed  with  which  neutrons  generated  by  the  first 
fissions  get  into  other  nuclei  and  produce  further  fission,  and  (b) 
the  speed  with  which  the  bomb  flies  apart.  Using  known  prin- 
ciples of  energy  generation,  temperature  and  pressure  rise,  and 
expansion  of  solids  and  vapors,  it  was  possible  to  estimate  the 
order  of  magnitude  of  the  time  interval  between  the  beginning 
and  end  of  the  nuclear  chain  reaction.  Almost  all  the  technical 
difficulties  of  the  project  come  from  the  extraordinary  brevity 
of  this  time  interval. 

12.10.  In  earlier  chapters  we  stated  that  no  self-sustaining 
chain  reaction  could  be  produced  in  a  block  of  pure  uranium 
metal,  no  matter  how  large,  because  of  parasitic  capture  of  the 
neutrons  by  U-238.  This  conclusion  has  been  borne  out  by  various 
theoretical  calculations  and  also  by  direct  experiment.  For  pur- 
poses of  producing  a  non-explosive  pile,  the  trick  of  using  a  lattice 
and  a  moderator  suffices — by  reducing  parasitic  capture  suffi- 
ciently. For  purposes  of  producing  an  explosive  unit,  however, 
it  turns  out  that  this  process  is  unsatisfactory  on  two  counts.  First, 
the  thermal  neutrons  take  so  long  (so  many  micro-seconds)  to 
act  that  only  a  feeble  explosion  would  result.  Second,  a  pile  is 
ordinarily  far  too  big  to  be  transported.  It  is  therefore  necessary 
to  cut  down  parasitic  capture  by  removing  the  greater  part  of  the 
U-238 — or  to  use  plutonium. 

12.11.  Naturally,  these  general  principles — and  others — had 
been  well  established  before  the  Los  Alamos  project  was  set  up. 


12.12.  The  calculation  of  the  critical  size  of  a  chain-reacting 
unit  is  a  problem  that  has  already  been  discussed  in  connection 
with  piles.  Although  the  calculation  is  simpler  for  a  homogeneous 
metal  unit  than  for  a  lattice,  inaccuracies  remained  in  the  course 
of  the  early  work,  both  because  of  lack  of  accurate  knowledge  of 

210  Work  on  the  Atomic  Bomb 

constants  and  because  of  mathematical  difficulties.  For  example, 
the  scattering,  fission,  and  absorption  cross  sections  of  the  nuclei 
involved  all  vary  with  neutron  velocity.  The  details  of  such  varia- 
tion were  not  known  experimentally  and  were  difficult  to  take 
into  account  in  making  calculations.  By  the  spring  of  1 943  several 
estimates  of  critical  size  had  been  made  using  various  methods  of 
calculation  and  using  the  best  available  nuclear  constants,  but 
the  limits  of  error  remained  large. 


12.13.  In  a  uranium-graphite  chain-reacting  pile  the  critical 
size  may  be  considerably  reduced  by  surrounding  the  pile  with 
a  layer  of  graphite,  since  such  an  envelope  "reflects"  many  neu- 
trons back  into  the  pile.  A  similar  envelope  can  be  used  to  reduce 
the  critical  size  of  the  bomb,  but  here  the  envelope  has  an  addi- 
tional role:  its  very  inertia  delays  the  expansion  of  the  reacting 
material.  For  this  reason  such  an  envelope  is  often  called  a 
tamper.  Use  of  a  tamper  clearly  makes  for  a  longer  lasting,  more 
energetic,  and  more  efficient  explosion.  The  most  effective  tamper 
is  the  one  having  the  highest  density;  high  tensile  strength  turns 
out  to  be  unimportant.  It  is  a  fortunate  coincidence  that  mate- 
rials of  high  density  are  also  excellent  as  reflectors  of  neutrons. 


12.14.  As  has  already  been  remarked,  the  bomb  tends  to  fly 
to  bits  as  the  reaction  proceeds  and  this  tends  to  stop  the  reaction. 
To  calculate  how  much  the  bomb  has  to  expand  before  the 
reaction  stops  is  relatively  simple.  The  calculation  of  how  long 
this  expansion  takes  and  how  far  the  reaction  goes  in  that  time 
is  much  more  difficult. 

12.15.  While  the  effect  of  a  tamper  is  to  increase  the  efficiency 
both  by  reflecting  neutrons  and  by  delaying  the  expansion  of 
the  bomb,  the  effect  on  the  efficiency  is  not  as  great  as  on  the 
critical  mass.  The  reason  for  this  is  that  the  process  of  reflection 
is  relatively  time-consuming   and   may  not  occur  extensively 
before  the  chain  reaction  is  terminated. 

Work  on  the  Atomic  Bomb  21 1 


12.16.  As  stated  in  Chapter  II,  it  is  impossible  to  prevent  a 
chain  reaction  from  occurring  when  the  size  exceeds  the  critical 
size.  For  there  are  always  enough  neutrons  (from  cosmic  rays, 
from  spontaneous  fission  reactions,   or  from  alpha-particle-in- 
duced reactions  in  impurities)  to  initiate  the  chain.  Thus  until 
detonation  is  desired,  the  bomb  must  consist  of  a  number  of 
separate  pieces  each  one  of  which  is  below  the  critical  size  either 
by  reason   of  small    size  or    unfavorable    shape.   To   produce 
detonation,  the  parts  of  the  bomb  must  be  brought  together 
rapidly.  In  the  course  of  this  assembly  process  the  chain  reaction 
is  likely  to  start — because  of  the  presence  of  stray  neutrons — 
before  the  bomb  has  reached  its  most  compact  (most  reactive) 
form.  Thereupon  the  explosion  tends  to  prevent  the  bomb  from 
reaching  that  most  compact  form.  Thus  it  may  turn  out  that 
the  explosion  is  so  inefficient  as  to  be  relatively  useless.  The 
problem,  therefore,  is  two-fold:  (1)  to  reduce  the  time  of  assembly 
to  a  minimum;  and  (2)  to  reduce  the  number  of  stray  (pre- 
detonation)  neutrons  to  a  minimum. 

12.17.  Some  consideration  was  given  to  the  danger  of  pro- 
ducing a  "dud"  or  a  detonation  so  inefficient  that  even  the  bomb 
itself  would  not  be  completely  destroyed.  This  would,  of  course, 
present  the  enemy  with  a  supply  of  highly  valuable  material. 


12.18.  In  Chapters  II  and  IV  it  was  pointed  out  that  the 
amount  of  energy  released  was  not  the  sole  criterion  of  the  value 
of  a  bomb.  There  was  no  assurance  that  one  uranium  bomb 
releasing  energy  equal  to  the  energy  released  by  20,000  tons  of 
TNT  would  be  as  effective  in  producing  military  destruction  as, 
say,  10,000  two-ton  bombs.  In  fact,  there  were  good  reasons  to 
believe  that  the  destructive  effect  per  calorie  released  decreases 
as  the  total  amount  of  energy  released  increases.  On  the  other 
hand,  in  atomic  bombs  the  total  amount  of  energy  released  per 
kilogram  of  fissionable  material   (i.e.,  the  efficiency  of  energy 

212  Work  on  the  Atomic  Bomb 

release)  increases  with  the  size  of  the  bomb.  Thus  the  optimum 
size  of  the  atomic  bomb  was  not  easily  determined.  A  tactical 
aspect  that  complicates  the  matter  further  is  the  advantage  of 
simultaneous  destruction  of  a  large  area  of  enemy  territory.  In 
a  complete  appraisal  of  the  effectiveness  of  an  atomic  bomb, 
attention  must  also  be  given  to  effects  on  morale.*  The  bomb 
is  detonated  in  combat  at  such  a  height  above  the  ground  as  to 
give  the  maximum  blast  effect  against  structures,  and  to  dis- 
seminate the  radioactive  products  as  a  cloud.  On  account  of  the 
height  of  the  explosion  practically  all  the  radioactive  products 
are  carried  upward  in  the  ascending  column  of  hot  air  and  dis- 
persed harmlessly  over  a  wide  area.  Even  in  the  New  Mexico 
test,  where  the  height  of  explosion  was  necessarily  low,  only  a 
very  small  fraction  of  the  radioactivity  was  deposited  immediately 
below  the  bomb. 


12.19.  Since  estimates  had  been  made  of  the  speed  that  would 
bring  together  subcritical  masses  of  U-235  rapidly  enough  to 
avoid  predetonation,  a  good  deal  of  thought  had  been  given  to 
practical  methods  of  doing  this.  The  obvious  method  of  very 
rapidly  assembling  an  atomic  bomb  was  to  shoot  one  part  as  a 
projectile  in  a  gun  against  a  second  part  as  a  target.  The  projectile 
mass,  projectile  speed,  and  gun  caliber  required  were  not  far 
from  the  range  of  standard  ordnance  practice,  but  novel  problems 
were  introduced  by  the  importance  of  achieving  sudden  and 
perfect  contact  between  projectile   and  target,   by  the  use  of 
tampers,  and  by  the  requirement  of  portability.  None  of  these 
technical  problems  had  been  studied  to  any  appreciable  extent 
prior  to  the  establishment  of  the  Los  Alamos  laboratory. 

12.20.  It  had  also  been  realized  that  schemes  probably  might 
be  devised  whereby  neutron  absorbers  could  be  incorporated  in 
the  bomb  in  such  a  way  that  they  would  be  rendered  less  effective 
by  the  initial  stages  of  the  chain  reactions.  Thus  the  tendency  for 

*  The  rest  of  this  paragraph  is  from  a  War  Department  release  subsequent 
to  the  first  use  of  atomic  bombs  against  Japan. 

Work  on  the  Atomic  Bomb  213 

the  bomb  to  detonate  prematurely  and  inefficiently  would  be 
minimized.  Such  devices  for  increasing  the  efficiency  of  the  bomb 
are  called  auto-catalytic. 


12.21.  In  April  1943  the  available  information  of  interest  in 
connection  with  the  design  of  atomic  bombs  was  preliminary 
and  inaccurate.  Further  and  extensive  theoretical  work  on  critical 
size,   efficiency,  effect  of  tamper,   method  of  detonation,   and 
effectiveness  was  urgently  needed.  Measurements  of  the  nuclear 
constants  of  U-235,  plutonium,  and  tamper  material  had  to  be 
extended  and  improved.  In  the  cases  of  U-235  and  plutonium, 
tentative   measurements   had   to   be   made   using   only  minute 
quantities  until  larger  quantities  became  available. 

12.22.  Besides  these  problems  in  theoretical  and  experimental 
physics,  there  was  a  host  of  chemical,  metallurgical,  and  technical 
problems  that  had  hardly  been  touched.   Examples  were  the 
purification  and  fabrication  of  U-235  and  plutonium,  and  the 
fabrication  of  the  tamper.  Finally,  there  were  problems  of  instan- 
taneous assembly  of  the  bomb  that  were  staggering  in  their 



12.23.  For  administrative  purposes  the  scientific  staff  at  Los 
Alamos  was  arranged  in  seven  divisions,  which  have  been  re- 
arranged at  various  times.  During  the  spring  of  1 945  the  divisions 
were:   Theoretical   Physics   Division   under   H.   Bethe,   Experi- 
mental Nuclear  Physics  Division  under  R.  R.  Wilson,  Chemistry 
and  Metallurgy  Division  under  J.  W.  Kennedy  and  C.  S.  Smith, 
Ordnance  Division  under  Capt.  W.  S.  Parsons  (USN),  Explosives 
Division  under  G.  B.  Kistiakowsky,  Bomb  Physics  Division  under 
R.  F.  Bacher,  and  an  Advanced  Development  Division  under 
E.   Fermi.  All  the  divisions  reported  to  J.   R.   Oppenheimer, 
Director  of  the  Los  Alamos  Laboratory  who  has  been  assisted  in 

214  Work  on  the  Atomic  Bomb 

coordinating  the  research  by  S.  K.  Allison  since  December  1944. 
J.  Chad  wick  of  England  and  N.  Bohr  of  Denmark  spent  a  great 
deal  of  time  at  Los  Alamos  and  gave  invaluable  advice.  Chadwick 
was  the  head  of  a  British  delegation  which  contributed  materially 
to  the  success  of  the  laboratory.  For  security  reasons,  most  of  the 
work  of  the  laboratory  can  be  described  only  in  part. 


12.24.  There    were    two    considerations    that    gave    unusual 
importance  to  the  work  of  the  Theoretical  Physics  Division  under 
H.  Bethe.  The  first  of  these  was  the  necessity  for  effecting  simul- 
taneous development  of  everything  from  the  fundamental  mate- 
rials to  the  method  of  putting  them  to  use — all  despite  the  virtual 
unavailability  of  the  principal  materials  (U-235  and  plutonium) 
and  the  complete  novelty  of  the  processes.  The  second  considera- 
tion was  the  impossibility  of  producing   (as  for  experimental 
purposes)  a  "small-scale"  atomic  explosion  by  making  use  of 
only  a  small  amount  of  fissionable  material.  (No  explosion  occurs 
at  all  unless  the  mass  of  the  fissionable  material  exceeds  the 
critical  mass.)  Thus  it  was  necessary  to  proceed  from  data  ob- 
tained in  experiments  on  infinitesimal  quantities  of  materials 
and  to  combine  it  with  the  available  theories  as  accurately  as 
possible  in  order  to  make  estimates  as  to  what  would  happen  in 
the  bomb.  Only  in  this  way  was  it  possible  to  make  sensible  plans 
for  the  other  parts  of  the  project,  and  to  make  decisions  on  design 
and  construction  without  waiting  for  elaborate  experiments  on 
large  quantities  of  material.  To  take  a  few  examples,  theoretical 
work  was  required  in  making  rough  determinations  of  the  dimen- 
sions of  the  gun,  in  guiding  the  metallurgists  in  the  choice  of 
tamper  materials,  and  in  determining  the  influence  of  the  purity 
of  the  fissionable  material  on  the  efficiency  of  the  bomb. 

12.25.  The  determination  of  the  critical  size  of  the  bomb  was 
one  of  the  main  problems  of  the  Theoretical  Physics  Division.  In 
the  course  of  time,   several  improvements  were  made  in  the 
theoretical  approach  whereby  it  was  possible  to  take  account  of 
practically  all  the  complex  phenomena  involved.  It  was  at  first 

Work  on  the  Atomic  Bomb  215 

considered  that  the  diffusion  of  neutrons  was  similar  to  the  diffu- 
sion of  heat,  but  this  naive  analogy  had  to  be  forsaken.  In  the 
early  theoretical  work  the  assumptions  were  made  that  the 
neutrons  all  had  the  same  velocity  and  all  were  scattered  iso- 
tropically.  A  method  was  thus  developed  which  permitted  cal- 
culation of  the  critical  size  for  various  shapes  of  the  fissionable 
material  provided  that  the  mean  free  path  of  the  neutrons  was 
the  same  in  the  tamper  material  as  in  the  fissionable  material. 
This  method  was  later  improved  first  by  taking  account  of  the 
angular  dependence  of  the  scattering  and  secondly  by  allowing 
for  difference  in  mean  free  path  in  core  and  tamper  materials. 
Still  later,  means  were  found  of  taking  into  account  the  effects 
of  the  distribution  in  velocity  of  the  neutrons,  the  variations  of 
cross  sections  with  velocity,  and  inelastic  scattering  in  the  core 
and  tamper  materials.  Thus  it  became  possible  to  compute 
critical  sizes  assuming  almost  any  kind  of  tamper  material. 

12.26.  The  rate  at  which  the  neutron  density  decreases  in 
bomb  models  which  are  smaller  than  the  critical  size  can  be  cal- 
culated, and  all  the  variables  mentioned  above  can  be  taken  into 
account.  The  rate  of  approach  to  the  critical  condition  as  the 
projectile  part  of  the  bomb  moves  toward  the  target  part  of  the 
bomb  has  been  studied  by  theoretical  methods.  Furthermore, 
the  best  distribution  of  fissionable  material  in  projectile  and  target 
was  determined  by  theoretical  studies. 

12.27.  Techniques  were  developed  for  dealing  with  set-ups  in 
which  the  number  of  neutrons  is  so  small  that  a  careful  statistical 
analysis  must  be  made  of  the  effects  of  the  neutrons.  The  most 
important  problem  in  this  connection  was  the  determination  of 
the  probability  that,  when  a  bomb  is  larger  than  critical  size, 
a  stray  neutron  will  start  a  continuing  chain  reaction.  A  related 
problem  was  the  determination  of  the  magnitude  of  the  fluctua- 
tions in  neutron  density  in  a  bomb  whose  size  is  close  to  the  critical 
size.  By  the  summer  of  1945  many  such  calculations  had  been 
checked  by  experiments. 

12.28.  A  great  deal  of  theoretical  work  was  done  on  the  equa- 
tion of  state  of  matter  at  the  high  temperatures  and  pressures  to 

216  Work  on  the  Atomic  Bomb 

be  expected  in  the  exploding  atomic  bombs.  The  expansion  of 
the  various  constituent  parts  of  the  bomb  during  and  after  the 
moment  of  chain  reaction  has  been  calculated.  The  effects  of 
radiation  have  been  investigated  in  considerable  detail. 

12.29.  Having  calculated  the  energy  that  is  released  in  the 
explosion  of  an  atomic  bomb,  one  naturally  wants  to  estimate 
the  military  damage  that  will  be  produced.  This  involves  analysis 
of  the  shock  waves  in  air  and  in  earth,  the  determination  of  the 
effectiveness  of  a  detonation  beneath  the  surface  of  the  ocean,  etc. 

12.30.  In  addition  to  all  the  work  mentioned  above,  a  con- 
siderable amount  of  work  was  done  in  evaluating  preliminary 
experiments.  Thus  an  analysis  was  made  of  the  back-scattering 
of  neutrons   by   the   various   tamper   materials   proposed.    An 
analysis  was  also  made  of  the  results  of  experiments  on  the 
multiplication  of  neutrons  in  subcritical  amounts  of  fissionable 


12.31.  The    experiments    performed    by    the    Experimental 
Nuclear  Physics  group  at  Los  Alamos  were  of  two  kinds:  "differ- 
ential" experiments  as  for  determining  the  cross  section  for  fission 
of  a  specific  isotope  by  neutrons  of  a  specific  velocity,  and  "in- 
tegral" experiments  as  for  determining  the  average  scattering  of 
fission  neutrons  from  an  actual  tamper. 

12.32.  Many  nuclear  constants  had  already  been  determined 
at  the  University  of  Chicago  Metallurgical  Laboratory  and  else- 
where, but  a  number  of  important  constants  were  still  undeter- 
mined— especially  those  involving  high  neutron  velocities.  Some 
of  the  outstanding  questions  were  the  following: 

1.  What  are  the  fission  cross  sections  of  U-234,  U-235,  U-238, 
Pu-239,  etc.?  How  do  they  vary  with  neutron  velocity? 

2.  What  are  the  elastic  scattering  cross  sections  for  the  same 
nuclei  (also  for  nuclei  of  tamper  materials)?  How  do  they  vary 
with  neutron  velocity? 

3.  What  are  the  inelastic  cross  sections  for  the  nuclei  referred 
to  above? 

Work  on  the  Atomic  Bomb  217 

4.  Wh'at  are  the  absorption  cross  sections  for  processes  other 
than  fission? 

5.  How  many  neutrons  are  emitted  per  fission  in  the  case  of 
each  of  the  nuclei  referred  to  above? 

6.  What  is  the  full  explanation  of  the  fact  that  the  number  of 
neutrons  emitted  per  fission  is  not  a  whole  number? 

7.  What  is  the  initial  energy  of  the  neutrons  produced  by 

8.  Does  the  number  or  energy  of  such  neutrons  vary  with  the 
speed  of  the  incident  neutrons? 

9.  Are  fission  neutrons  emitted  immediately? 

10.  WTiat  is  the  probability  of  spontaneous  fission  of  the  various 
fissionable  nuclei? 

12.33.  In  addition  to  attempting  to  find  the  answers  to  these 
questions  the  Los  Alamos  Experimental  Nuclear  Physics  Division 
investigated  many  problems  of  great  scientific  interest  which 
were  expected  to  play  a  role  in  their  final  device.  Whether  or  not 
this  turned  out  to  be  the  case,  the  store  of  knowledge  thus  accumu- 
lated by  the  Division  forms  an  integral  and  invaluable  part  of  all 
thinking  on  nuclear  problems. 

12.34.  Experimental  Methods.  The  earlier  chapters  contain  little 
or  no  discussion  of  experimental  techniques  except  those  for  the 
observing  of  fast  (charged)  particles  (See  Appendix  1.).  To  obtain 
answers  to  the  ten  questions  posed  above,  we  should  like  to  be 
able  to: 

(1)  determine  the  number  of  neutrons  of  any  given  energy; 

(2)  produce  neutrons  of  any  desired  energy; 

(3)  determine  the  angles  of  deflection  of  scattered  neutrons; 

(4)  determine  the  number  of  fissions  occurring; 

(5)  detect  other  consequences  of  neutron  absorption,  e.g., 
artificial  radioactivity. 

We  shall  indicate  briefly  how  such  observations  are  made. 

12.35.  Detection  of  Neutrons.  There  are  three  ways  in  which 
neutrons  can  be  detected:  by  the  ionization  produced  by  light 
atomic  nuclei  driven  forward  at  high  speeds  by  elastic  collisions 

218  Work  on  the  Atomic  Bomb 

with  neutrons,  by  the  radioactive  disintegration  of  unstable 
nuclei  formed  by  the  absorption  of  neutrons,  and  by  fission 
resulting  from  neutron  absorption.  All  three  processes  lead  to 
the  production  of  ions  and  the  resulting  ionization  may  be 
detected  using  electroscopes,  ionization  chambers,  Geiger-Miiller 
counters,  Wilson  cloud  chambers,  tracks  in  photographic  emul- 
sion, etc. 

12.36.  While  the  mere  detection  of  neutrons  is  not  difficult, 
the  measurement  of  the  neutron  velocities  is  decidedly  more  so. 
The    Wilson    cloud    chamber    method    and    the    photographic 
emulsion  method  give  the  most  direct  results  but  are  tedious  to 
apply.  More  often  various  combinations  of  selective  absorbers 
are  used.  Thus,  for  example,  if  a  foil  known  to  absorb  neutrons 
of  only  one  particular  range  of  energies  is  inserted  in  the  path  of 
the  neutrons  and  is  then  removed,  its  degree  of  radioactivity  is 
presumably   proportional   to   the   number   of  neutrons   in   the 
particular  energy  range  concerned.  Another  scheme  is  to  study 
the  induced  radioactivity  known  to  be  produced  only  by  neutrons 
whose  energy  lies  above  a  certain  threshold. 

12.37.  One  elegant  scheme  for  studying  the  effects  of  neutrons 
of  a  single,  arbitrarily-selected  velocity  is  the  "time  of  flight" 
method.  In  this  method  a  neutron  source  is  modulated,  i.e.,  the 
source  is  made  to  emit  neutrons  in  short  "bursts"  or  "pulses." 
In  each  pulse  there  are  a  great  many  neutrons — of  a  very  wide 
range  of  velocities.  The  target  material  and  the  detector  are 
situated  a  considerable  distance  from  the  source  (several  feet 
or  yards  from  it).  The  detector  is  "modulated"  also,  and  with 
the  same  periodicity.  The  timing  or  phasing  is  made  such  that 
the  detector  is  responsive  only  for  a  short  interval  beginning  a 
certain  time  after  the  pulse  of  neutrons  leaves  the  source.  Thus 
any  effects  recorded  by  the  detector  (e.g.,  fissions  in  a  layer  of 
uranium  deposited  on  an  inner  surface  of  an  ionization  chamber) 
are  the  result  only  of  neutrons  that  arrive  just  at  the  moment  of 
responsivity  and  therefore  have  travelled  from  the  source  in  a 
certain  time  interval.  In  other  words,  the  measured  effects  are 
due  only  to  the  neutrons  having  the  appropriate  velocity. 

Work  on  the  Atomic  Bomb  219 

12.38.  Production  of  Neutrons.  All  neutrons  are  produced  as  the 
result  of  nuclear  reactions,  and  their  initial  speed  depends  on  the 
energy  balance  of  the  particular  reaction.    If  the  reaction  is 
endothermic,  that  is,  if  the  total  mass  of  the  resultant  particles  is 
greater  than  that  of  the  initial  particles,  the  reaction  does  not 
occur  unless  the  bombarding  particle  has  more  than  the  "thresh- 
old"' kinetic  energy.  At  higher  bombarding  energies  the  kinetic 
energy-  of  the  resulting  particles,  specifically  of  the  neutrons,  goes 
up  with  the  increase  of  kinetic  energy  of  the  bombarding  particle 
above  the  threshold  value.  Thus  the  Li7(p,  n)Be7  reaction  absorbs 
1.6  Mev  energy  since  the  product  particles  are  heavier  than  the 
initial  particles.  Any  further  energy  of  the  incident  protons  goes 
into  kinetic  energy  of  the  products  so  that  the  maximum  speed 
of  the  neutrons  produced  goes  up  with  the  speed  of  the  incident 
protons.  However,  to  get  neutrons  of  a  narrow  range  of  speed,  a 
thin  target  must  be  used,  the  neutrons  must  all  come  off  at  the 
same  angle,  and  the  protons  must  all  strike  the  target  with  the 
same  speed. 

12.39.  Although  the  same  energy  and  momentum  conserva- 
tion  laws  apply  to  exothermic  nuclear  reactions,   the  energy 
release  is  usually  large  compared  to  the  kinetic  energy  of  the 
bombarding  particles  and  therefore  essentially  determines  the 
neutron  speed.    Often  there  are  several  ranges  of  speed  from 
the  same  reaction.  There  are  some  reactions  that  produce  very 
high  energy  neutrons  (nearly  15  Mev). 

12.40.  Since  there  is  a  limited  number  of  nuclear  reactions 
usable  for  neutron  sources,   there  are   only  certain  ranges  of 
neutron  speeds  that  can  be  produced  originally.  There  is  no 
difficulty  about  slowing  down  neutrons,  but  it  is  impossible  to 
slow  them  down  uniformly,  that  is,  without  spreading  out  the 
velocity  distribution.  The  most  effective  slowing-down  scheme  is 
the  use  of  a  moderator,  as  in  the  graphite  pile;  in  fact,  the  pile 
itself  is  an  excellent  source  of  thermal  (i.e.,  very  low  speed)  or 
nearly  thermal  neutrons. 

12.41.  Determination  of  Angles  of  Deflection.  The  difficulties  in 
measuring  the  angles  of  deflection  of  neutrons  are  largely  of 

220  Work  on  the  Atomic  Bomb 

intensity  and  interpretation.  The  number  of  neutrons  scattered 
in  a  particular  direction  may  be  relatively  small,  and  the  "scat- 
tered" neutrons  nearly  always  include  many  strays  not  coming 
from  the  intended  target. 

12.42.  Determination  of  Number  of  Fissions.  The  determination 
of  the  number  of  fissions  which  are  produced  by  neutrons  or 
occur  spontaneously  is  relatively  simple.  lonization  chambers, 
counter  tubes,  and  many  other  types  of  detectors  can  be  used. 

12.43.  Detection  of  Products  of  Capture  of  Neutrons.  Often  it  is 
desirable  to  find  in  detail  what  has  happened  to  neutrons  that 
are  absorbed  but  have  not  produced  fission,  e.g.,  resonance  or 
"radiative"  capture  of  neutrons  by  U-238  to  form  U-239  which 
leads   to   the   production   of  plutonium.    Such   studies   usually 
involve  a  combination  of  microchemical  separations  and  radio- 
activity analyses. 

12.44.  Some  Experiments  on  Nuclear  Constants.  By  the  time  that 
the  Los  Alamos  laboratory  had  been  established,  a  large  amount 
of  work  had  been  done  on  the  effects  of  slow  neutrons  on  the 
materials   then   available.    For   example,    the   thermal-neutron 
fission  cross  section  of  natural  uranium  had  been  evaluated,  and 
similarly  for  the  separated  isotopes  of  uranium  and  for  plutonium. 
Some  data  on  high-speed-neutron  fission  cross  sections  had  been 
published,  and  additional  information  was  available  in  project 
laboratories.  To  extend  and  improve  such  data,  Los  Alamos  per- 
fected the  use  of  the  Van  de  Graaff  generator  for  the  Li7(p,  n)Be7 
reaction,  so  as  to  produce  neutrons  of  any  desired  energy  lying 
in  the  range  from  3,000  electron  volts  to  two  million  electron 
volts.   Success  was  also  achieved  in  modulating  the  cyclotron 
beam    and    developing   the   neutron    time-of-flight   method    to 
produce  effects  of  many  speed  intervals  at  once.  Special  methods 
were  devised  for  filling  in  the  gaps  in  neutron  energy  range. 
Particularly  important  was  the  refinement  of  measurement  made 
possible  as  greater  quantities  of  U-235,  U-238  and  plutonium 
began  to  be  received.  On  the  whole,  the  value  of  the  cross  section 
for  fission  as  a  function  of  neutron  energy  from  practically  zero 
electron  volts  to  three  million  electron  volts  is  now  fairly  well 
known  for  these  materials. 

Work  on  the  Atomic  Bomb  221 

12.45.  Some  Integral  Experiments.  Two  "integral  experiments" 
(experiments   on   assembled   or   integrated   systems   comprising 
fissionable  material,  reflector,  and  perhaps  moderator  also)  may 
be  described.  In  the  first  of  these  integral  experiments  a  chain- 
reacting  system  was   constructed  which  included   a  relatively 
large  amount  of  U-235  in  liquid  solution.  It  was  designed  to 
operate  at  a  very  low  power  level,  and  it  had  no  cooling  system. 
Its  purpose  was  to  provide  verification  of  the  effects  predicted 
for  reacting  systems  containing  enriched  U-235.  The  results  were 
very  nearly  as  expected. 

12.46.  The  second  integral  experiment  was  carried  out  on  a 
pile  containing  a  mixture  of  uranium  and  a  hydrogenous  moder- 
ator. In  this  first  form,  the  pile  was  thus  a  slow-neutron  chain- 
reacting  pile.  The  pile  was  then  rebuilt  using  less  hydrogen.  In 
this  version  of  the  pile,  fast-neutron  fission  became  important. 
The  pile  was  rebuilt  several  more  times,  less  hydrogen  being 
used  each  time.  By  such  a  series  of  reconstructions,  the  reaction 
character  was  successively  altered,  so  that  thermal  neutron  fission 
became  less  and  less  important  while  fast  neutron  fission  became 
more  and  more  important — approaching  the  conditions  to  be 
found  in  the  bomb. 

12.47.  Summary  of  Results  on  Nuclear  Physics.  The  nuclear  con- 
stants of  U-235,  U-238,  and  plutonium  have  been  measured  with 
a  reasonable  degree  of  accuracy  over  the  range  of  neutron  energies 
from  thermal  to  three  million  electron  volts.  In  other  words, 
questions  1,  2,  3,  4,  and  5  of  the  ten  questions  posed  at  the 
beginning  of  this  section  have  been  answered.  The  fission  spectrum 
(question  7)  for  U-235  and  Pu-239  is  reasonably  well  known. 
Spontaneous  fission  (question  10)  has  been  studied  for  several 
types  of  nuclei.  Preliminary  results  on  questions  6,  8,  and  9, 
involving  details  of  the  fission  process,  have  been  obtained. 


12.48.  The  Chemistry  and  Metallurgy  Division  of  the  Los 
Alamos   Laboratory   was   under   the  joint   direction   of  J.    W. 
Kennedy  and  C.  S.  Smith.  It  was  responsible  for  final  purification 
of  the  enriched  fissionable  materials,  for  fabrication  of  the  bomb 

222  Work  on  the  Atomic  Bomb 

core,  tamper,  etc.,  and  for  various  other  matters.  In  all  this 
division's  work  on  enriched  fissionable  materials  especial  care 
had  to  be  taken  not  to  lose  any  appreciable  amounts  of  the  ma- 
terials which  are  worth  much  more  than  gold.  Thus  the  pro- 
cedures already  well-established  at  Chicago  and  elsewhere  for 
purifying  and  fabricating  natural  uranium  were  often  not  satis- 
factory for  handling  highly-enriched  samples  of  U-235. 


12.49.  The  above  account  of  the  work  of  the  Theoretical 
Physics,    Experimental    Nuclear    Physics,    and    Chemistry   and 
Metallurgy    Divisions    is    very    incomplete    because    important 
aspects  of  this  work  cannot  be  discussed  for  reasons  of  security. 
For  the  same  reasons  none  of  the  work  of  the  Ordnance,  Explo- 
sives, and  Bomb  Physics  Divisions  can  be  discussed  at  all. 


12.50.  In  the  spring  of  1943  an  entirely  new  laboratory  was 
established  at  Los  Alamos,  New  Mexico,  under  J.  R.  Oppen- 
heimer  for  the  purpose  of  investigating  the  design  and  construc- 
tion of  the  atomic  bomb,  from  the  stage  of  receipt  of  U-235  or 
plutonium  to  the  stage  of  use  of  the  bomb.  The  new  laboratory 
improved  the  theoretical  treatment  of  design  and  performance 
problems,  refined  and  extended  the  measurements  of  the  nuclear 
constants  involved,  developed  methods  of  purifying  the  materials 
to  be  used,  and,  finally,  designed  and  constructed  operable  atomic 



13.1.  As  the  result  of  the  labors  of  the  Manhattan  District 
organization  in  Washington  and  in  Tennessee,  of  the  scientific 
groups  at  Berkeley,  Chicago,  Columbia,  Los  Alamos,  and  else- 
where, of  the  industrial  groups  at  Clinton,  Hanford,  and  many 
other  places,  the  end  of  June  1945  finds  us  expecting  from  day  to 
day  to  hear  of  the  explosion  of  the  first  atomic  bomb  devised  by 
man.  All  the  problems  are  believed  to  have  been  solved  at  least 
well  enough  to  make  a  bomb  practicable.  A  sustained  neutron 
chain  reaction  resulting  from  nuclear  fission  has  been  demon- 
strated; the  conditions  necessary  to  cause  such  a  reaction  to 
occur  explosively  have  been  established  and  can  be  achieved; 
production  plants  of  several  different  types  are  in  operation, 
building  up  a  stock  pile  of  the  explosive  material.  Although  we 
do  not  know  when  the  first  explosion  will  occur  nor  how  effective 
it  will  be,  announcement  of  its  occurrence  will  precede  the  pub- 
lication of  this  report.   Even  if  the  first  attempt  is  relatively 
ineffective,  there  is  little  doubt  that  later  efforts  will  be  highly 
effective;  the  devastation  from  a  single  bomb  is  expected  to  be 
comparable  to  that  of  a  major  air  raid  by  usual  methods. 

13.2.  A  weapon  has  been  developed  that  is  potentially  destruc- 
tive beyond  the  wildest  nightmares  of  the  imagination;  a  wreapon 
so  ideally  suited  to  sudden  unannounced  attack  that  a  country's 
major   cities   might   be   destroyed   overnight   by   an   ostensibly 
friendly  power.  This  weapon  has  been  created  not  by  the  devilish 
inspiration  of  some  warped  genius  but  by  the  arduous  labor  of 
thousands  of  normal  men  and  women  working  for  the  safety  of 
their  country.  Many  of  the  principles  that  have  been  used  were 
well  known  to  the  international  scientific  world  in  1940.  To 
develop  the  necessary  industrial  processes  from  these  principles 


224  General  Summary 

has  been  costly  in  time,  effort,  and  money,  but  the  processes 
which  we  selected  for  serious  effort  have  worked  and  several 
that  we  have  not  chosen  could  probably  be  made  to  work.  We 
have  an  initial  advantage  in  time  because,  so  far  as  we  know, 
other  countries  have  not  been  able  to  carry  out  parallel  develop- 
ments during  the  war  period.  We  also  have  a  general  advantage 
in  scientific  and  particularly  in  industrial  strength,  but  such  an 
advantage  can  easily  be  thrown  away. 

13.3.  Before  the  surrender  of  Germany  there  was  always  a 
chance  that  German  scientists  and  engineers  might  be  developing 
atomic  bombs  which  would  be  sufficiently  effective  to  alter  the 
course  of  the  war.  There  was  therefore  no  choice  but  to  work  on 
them  in  this  country.  Initially  many  scientists  could  and  did  hope 
that  some   principle  would  emerge  which  would   prove   that 
atomic  bombs  were  inherently  impossible.  This  hope  has  faded 
gradually;  fortunately  in  the  same  period  the  magnitude  of  the 
necessary  industrial  effort  has  been  demonstrated  so  that  the  fear 
of  German  success  weakened  even  before  the  end  came.  By  the 
same  token,  most  of  us  are  certain  that  the  Japanese  cannot 
develop  and  use  this  weapon  effectively. 


13.4.  As  to  the  future,  one  may  guess  that  technical  develop- 
ments will  take  place  along  two  lines.  From  the  military  point 
of  view  it  is  reasonably  certain  that  there  will  be  improvements 
both  in  the  processes  of  producing  fissionable  material  and  in  its 
use.  It  is  conceivable  that  totally  different  methods  may  be  dis- 
covered for  converting  matter  into  energy  since  it  is  to  be  remem- 
bered that  the  energy  released  in  uranium  fission  corresponds  to 
the  utilization  of  only  about  one-tenth  of  one  per  cent  of  its  mass. 
Should  a  scheme  be  devised  for  converting  to  energy  even  as 
much  as  a  few  per  cent  of  the  matter  of  some  common  material, 
civilization  would  have  the  means  to  commit  suicide  at  will. 

13.5.  The  possible  uses  of  nuclear  energy  are  not  all  destruc- 
tive, and  the  second  direction  in  which  technical  development  can 
be  expected  is  along  the  paths  of  peace.  In  the  fall  of  1 944  General 

General  Summary  225 

Groves  appointed  a  committee  to  look  into  these  possibilities  as 
well  as  those  of  military  significance.  This  committee  (Dr.  R.  C. 
Tolman,  chairman;  Rear  Admiral  E.  W.  Mills  (USN)  with 
Captain  T.  A.  Solberg  (USN)  as  deputy,  Dr.  W.  K.  Lewis,  and 
Dr.  H.  D.  Smyth)  received  a  multitude  of  suggestions  from  men 
on  the  various  projects,  principally  along  the  lines  of  the  use  of 
nuclear  energy  for  power  and  the  use  of  radioactive  by-products 
for  scientific,  medical,  and  industrial  purposes.  While  there  was 
general  agreement  that  a  great  industry  might  eventually  arise, 
comparable,  perhaps,  with  the  electronics  industry,  there  was 
disagreement  as  to  how  rapidly  such  an  industry  would  grow;  the 
consensus  was  that  the  growth  would  be  slow  over  a  period  of 
many  years.  At  least  there  is  no  immediate  prospect  of  running 
cars  with  nuclear  power  or  lighting  houses  with  radioactive 
lamps  although  there  is  a  good  probability  that  nuclear  power 
for  special  purposes  could  be  developed  within  ten  years  and  that 
plentiful  supplies  of  radioactive  materials  can  have  a  profound 
effect  on  scientific  research  and  perhaps  on  the  treatment  of 
certain  diseases  in  a  similar  period. 


13.6.  During  the  war  the  effort  has  been  to  achieve  the  maxi- 
mum military  results.  It  has  been  apparent  for  some  time  that 
some  sort  of  government  control  and  support  in  the  field  of 
nuclear  energy  must  continue  after  the  war.  Many  of  the  men 
associated  with  the  project  have  recognized  this  fact  and  have 
come  forward  with  various  proposals,  some  of  which  were  con- 
sidered by  the  Tolman  Committee,  although  it  was  only  a 
temporary  advisory  committee  reporting  to  General  Groves.  An 
interim  committee  at  a  high  level  is  now  engaged  in  formulating 
plans  for  a  continuing  organization.  This  committee  is  also 
discussing  matters  of  general  policy  about  which  many  of  the 
more  thoughtful  men  on  the  project  have  been  deeply  concerned 
since  the  work  was  begun  and  especially  since  success  became 
more  and  more  probable. 

226  General  Summary 


13.7.  We  find  ourselves  with  an  explosive  which  is  far  from 
completely  perfected.  Yet  the  future  possibilities  of  such  explosives 
are  appalling,  and  their  effects  on  future  wars  and  international 
affairs  are  of  fundamental  importance.  Here  is  a  new  tool  for 
mankind,  a  tool  of  unimaginable  destructive  power.  Its  develop- 
ment raises  many  questions  that  must  be  answered  in  the  near 

13.8.  Because  of  the  restrictions  of  military  security  there  has 
been  no  chance  for  the  Congress  or  the  people  to  debate  such 
questions.  They  have  been  seriously  considered  by  all  concerned 
and  vigorously  debated  among  the  scientists,  and  the  conclusions 
reached  have  been  passed  along  to  the  highest  authorities.  These 
questions  are  not  technical  questions;  they  are  political  and  social 
questions,  and  the  answers  given  to  them  may  affect  all  mankind 
for  generations.  In  thinking  about  them  the  men  on  the  project 
have  been  thinking  as  citizens  of  the  United  States  vitally  inter- 
ested in  the  welfare  of  the  human  race.  It  has  been  their  duty 
and  that  of  the  responsible  high  government  officials  who  were 
informed  to  look  beyond  the  limits  of  the  present  war  and  its 
weapons  to  the  ultimate  implications  of  these  discoveries.  This 
was  a  heavy  responsibility.   In  a  free  country  like  ours,  such 
questions  should  be  debated  by  the  people  and  decisions  must 
be  made  by  the  people  through  their  representatives.  This  is  one 
reason  for  the  release  of  this  report.  It  is  a  semi-technical  report 
which  it  is  hoped  men  of  science  in  this  country  can  use  to  help 
their  fellow  citizens  in  reaching  wise  decisions.  The  people  of 
the  country  must  be  informed  if  they  are  to  discharge  their 
responsibilities  wisely. 


In  Chapter  I  we  pointed  out  the  importance  of  ionization  in  the 
study  of  radioactivity  and  mentioned  the  electroscope.  In  this 
appendix  we  shall  mention  one  method  of  historical  importance 
comparable  with  the  electroscope  but  no  longer  used,  and  then 
we  shall  review  the  various  methods  now  in  use  for  observing 
alpha  particles,  beta  particles  (or  positrons),  gamma  rays,  and 
neutrons,  or  their  effects. 


The  closest  approach  that  can  be  made  to  "seeing"  an  atom  is 
to  see  the  bright  flash  of  light  that  an  alpha  particle  or  high-speed 
proton  makes  when  it  strikes  a  fluorescent  screen.  All  that  is 
required  is  a  piece  of  glass  covered  with  zinc  sulphide,  a  low- 
power  microscope,  a  dark  room,  a  well-rested  eye,  and  a  source 
of  alpha  particles.  Most  of  Rutherford's  famous  experiments, 
including  that  mentioned  in  paragraph  1.17,  involved  "counting" 
scintillations  but  the  method  is  tedious  and,  as  far  as  the  author 
knows,  has  been  entirely  superseded  by  electrical  methods. 


When  a  high-speed  charged  particle  like  an  alpha  particle  or  a 
high-speed  electron  passes  through  matter,  it  disrupts  the  mole- 
cules that  it  strikes  by  reason  of  the  electrical  forces  between  the 
charged  particle  and  the  electrons  in  the  molecule.  If  the  material 
is  gaseous,  the  resultant  fragments  or  ions  may  move  apart  and, 
if  there  is  an  electric  field  present,  the  electrons  knocked  out  of 
the  molecules  move  in  one  direction  and  the  residual  positive 
ions  in  another  direction.  A  beta  particle  with  a  million  electron 
volts  energy  will  produce  some  18,000  ionized  atoms  before  it  is 


228  Appendix  1 

stopped  completely  since  on  the  average  it  uses  up  about  60  volts 
energy  in  each  ionizing  collision.  Since  each  ionization  process 
gives  both  a  positive  and  a  negative  ion,  there  is  a  total  of  36,000 
charges  set  free  by  one  high-speed  electron,  but  since  each  charge 
is  only  1.6  X  10~19  coulomb,  the  total  is  only  about  6  X  10~15 
coulomb  and  is  still  very  minute.  The  best  galvanometer  can  be 
made  to  measure  a  charge  of  about  10~10  coulomb.  It  is  posssible 
to  push  the  sensitivity  of  an  electrometer  to  about  10~16  coulomb, 
but  the  electrometer  is  a  very  inconvenient  instrument  to  use. 

An  alpha  particle  produces  amounts  of  ionization  comparable 
with  the  beta  particle.  It  is  stopped  more  rapidly,  but  it  produces 
more  ions  per  unit  of  path.  A  gamma  ray  is  much  less  efficient 
as  an  ionizer  since  the  process  is  quite  different.  It  does  occa- 
sionally set  free  an  electron  from  a  molecule  by  Compton  scatter- 
ing or  the  photoelectric  effect,  and  this  secondary  electron  has 
enough  energy  to  produce  ionization.  A  neutron,  as  we  have 
already  mentioned  in  the  text,  produces  ionization  only  indirectly 
by  giving  high  velocity  to  a  nucleus  by  elastic  collision,  or  by 
disrupting  a  nucleus  with  resultant  ionization  by  the  fragments. 

If  we  are  to  detect  the  ionizing  effects  of  these  particles,  we 
must  evidently  use  the  resultant  effect  of  a  great  many  particles 
or  have  very  sensitive  means  of  measuring  electric  currents. 


Essentially  the  electroscope  determines  to  what  degree  the  air 
immediately  around  it  has  become  conducting  as  the  result  of 
the  ions  produced  in  it. 

The  simplest  form  of  electroscope  is  a  strip  of  gold  leaf  a  few 
centimeters  long,  suspended  by  a  hinge  from  a  vertical  insulated 
rod.  If  the  rod  is  charged,  the  gold  leaf  also  takes  up  the  same 
charge  and  stands  out  at  an  angle  as  a  result  of  the  repulsion  of 
like  charges.  As  the  charge  leaks  away,  the  leaf  gradually  swings 
down  against  the  rod,  and  the  rate  at  which  it  moves  is  a  measure 
of  the  conductivity  of  the  air  surrounding  it. 

A  more  rugged  form  of  electroscope  was  devised  by  C.  C. 
Lauritsen,  who  substituted  a  quartz  fiber  for  the  gold  leaf  and 

Observing  Fast  Particles  229 

used  the  elasticity  of  the  fiber  as  the  restoring  force  instead  of 
gravity.  The  fiber  is  made  conducting  by  a  thin  coating  of  metal. 
Again  the  instrument  is  charged,  and  the  fiber,  after  initial 
deflection,  gradually  comes  back  to  its  uncharged  position.  The 
position  of  the  fiber  is  read  in  a  low-power  microscope.  These 
instruments  can  be  made  portable  and  rugged  and  fairly  sensi- 
tive. They  are  the  standard  field  instrument  for  testing  the  level 
of  gamma  radiation,  particularly  as  a  safeguard  against  dangerous 


An  ionization  chamber  measures  the  total  number  of  ions 
produced  directly  in  it.  It  usually  consists  of  two  plane  electrodes 
between  which  there  is  a  strong  enough  electric  field  to  draw  all 
the  ions  to  the  electrodes  before  they  recombine  but  not  strong 
enough  to  produce  secondary  ions  as  in  the  instruments  we  shall 
describe  presently. 

By  careful  design  and  the  use  of  sensitive  amplifiers  an  ioniza- 
tion chamber  can  measure  a  number  of  ions  as  low  as  that  pro- 
duced by  a  single  alpha  particle,  or  it  can  be  used  much  like  an 
electroscope  to  measure  the  total  amount  of  ionizing  radiation 
present  instantaneously,  or  it  can  be  arranged  to  give  the  total 
amount  of  ionization  that  has  occurred  over  a  period  of  time. 


While  ionization  chambers  can  be  made  which  will  respond 
to  single  alpha  particles,  it  is  far  more  convenient  to  use  a  self- 
amplifying  device,  that  is,  to  make  the  ions  originally  produced 
make  other  ions  in  the  same  region  so  that  the  amplifier  circuits 
need  not  be  so  sensitive. 

In  a  proportional  counter  one  of  the  electrodes  is  a  fine  wire 
along  the  axis  of  the  second  electrode,  which  is  a  hollow  cylinder. 
The  effect  of  the  wire  is  to  give  strong  electric  field  strengths  close 
to  it  even  for  relatively  small  potential  differences  between  it 
and  the  other  electrode.  This  strong  field  quickly  accelerates  the 

230  Appendix  1 

primary  ions  formed  by  the  alpha  or  beta  particle  or  photon,  and 
these  accelerated  primary  ions  (particularly  the  electrons)  in 
turn  form  secondary  ions  in  the  gas  with  which  the  counter  is 
filled  so  that  the  total  pulse  of  current  is  much  increased. 

It  is  possible  to  design  and  operate  such  counters  in  such  a  way 
that  the  total  number  of  ions  formed  is  proportional  to  the  num- 
ber of  primary  ions  formed.  Thus  after  amplification  a  current 
pulse  can  be  seen  on  an  oscilloscope,  the  height  of  which  will 
indicate  how  effective  an  ionizer  the  initial  particle  was.  It  is 
quite  easy  to  distinguish  in  this  way  between  alpha  particles  and 
beta  particles  and  photons,  and  the  circuits  can  be  arranged  to 
count  only  the  pulses  of  greater  than  a  chosen  magnitude.  Thus 
a  proportional  counter  can  count  alpha  particles  against  a  back- 
ground of  betas  or  can  even  count  only  the  alpha  particles  having 
more  than  a  certain  energy. 


If  the  voltage  on  a  proportional  counter  is  raised,  there  comes 
a  point  when  the  primary  ions  from  a  single  alpha  particle,  beta 
particle,  or  photon  will  set  off  a  discharge  through  the  whole 
counter,  not  merely  multiply  the  number  of  primary  ions  in  the 
region  where  they  are  produced.  This  is  a  trigger  action  and  the 
current  is  independent  of  the  number  of  ions  produced ;  further- 
more, the  current  would  continue  indefinitely  if  no  steps  were 
taken  to  quench  it.  Quenching  can  be  achieved  entirely  by 
arranging  the  external  circuits  so  that  the  voltage  drops  as  soon 
as  current  passes  or  by  using  a  mixture  of  gases  in  the  counter 
which  "poison"  the  electrode  surface  as  soon  as  the  discharge 
passes  and  temporarily  prevent  the  further  emission  of  electrons, 
or  by  combining  both  methods. 

The  Geiger-Muller  counter  was  developed  before  the  propor- 
tional counter  and  remains  the  most  sensitive  instrument  for 
detecting  ionizing  radiation,  but  all  it  does  is  "count"  any  ionizing 
radiation  that  passes  through  it  whether  it  be  an  alpha  particle, 
proton,  electron,  or  photon. 

Observing  Fast  Particles  231 


It  is  one  thing  to  describe  the  principles  of  various  ionization 
chambers,  counters,  and  the  like;  quite  another  to  construct  and 
operate  them  successfully. 

First  of  all,  the  walls  of  the  counter  chamber  must  allow  the 
particles  to  enter  the  counter.  For  gamma  rays  this  is  a  minor 
problem,  but  for  relatively  low-speed  electrons  or  positrons  or  for 
alpha  particles  the  walls  of  the  counter  must  be  very  thin  or 
there  must  be  thin  windows. 

Then  there  are  great  variations  in  the  details  of  the  counter 
itself,  spacing  and  size  of  electrodes,  nature  of  the  gas  filling  the 
chamber,  its  pressure,  and  so  on. 

Finally,  the  interpretation  of  the  resultant  data  is  a  tricky 
business.  The  absorption  of  the  counter  walls  and  of  any  external 
absorbers  must  be  taken  into  account;  the  geometry  of  the 
counter  with  relation  to  the  source  must  be  estimated  to  translate 
observed  counts  into  actual  number  of  nuclear  events;  last  but 
not  always  least,  statistical  fluctuations  must  be  considered  since 
all  nuclear  reactions  are  governed  by  probability  laws. 


There  is  one  method  of  observing  nuclear  particles  that  depends 
directly  on  ionization  but  is  not  an  electrical  method.  It  uses  the 
fact  that  supersaturated  vapor  will  condense  more  readily  on 
ions  than  on  neutral  molecules.  If  air  saturated  with  water  vapor 
is  cooled  by  expansion  just  after  an  alpha  particle  has  passed 
through  it,  tiny  drops  of  water  condense  on  the  ions  formed  by 
the  alpha  particle  and  will  reflect  a  bright  light  strongly  enough 
to  be  seen  or  photographed  so  that  the  actual  path  of  the  alpha 
particle  is  recorded. 

This  method  developed  by  C.  T.  R.  Wilson  in  Cambridge, 
England,  about  1912  has  been  enormously  useful  in  studying  the 
behavior  of  individual  particles,  alphas,  protons,  electrons, 
positrons,  mesotrons,  photons,  and  the  fast  atoms  caused  by 
collisions  with  alphas,  protons,  or  neutrons.  Unlike  the  scintilla- 

232  Appendix  1 

tion  method,  its  companion  tool  for  many  years,  it  has  not  been 
superseded  and  is  still  used  extensively,  particularly  to  study 
details  of  collisions  between  nuclear  particles  and  atoms. 


The  tracks  of  individual  particles  passing  through  matter  can 
also  be  observed  in  photographic  emulsions,  but  the  lengths  of 
path  are  so  small  that  they  must  be  observed  under  a  microscope, 
where  they  appear  as  a  series  of  developed  grains  marking  the 
passage  of  the  particle.  This  method  of  observation  requires 
practically  no  equipment  but  is  tedious  and  of  limited  usefulness. 
It  is  possible,  however,  to  use  the  general  blackening  of  a 
photographic  film  as  a  measure  of  total  exposure  to  radiation, 
a  procedure  that  has  been  used  to  supplement  or  to  replace 
electroscopes  for  safety  control  in  many  parts  of  the  project. 


None  of  the  methods  we  have  described  is  directly  applicable 
to  neutrons,  but  all  of  them  are  indirectly  applicable  since  neu- 
trons produce  ions  indirectly.  This  happens  in  two  ways — by 
elastic  collision  and  by  nuclear  reaction.  As  we  have  already 
described,  a  fast  neutron  in  passing  through  matter  occasionally 
approaches  an  atomic  nucleus  so  closely  as  to  impart  to  it  a  large 
amount  of  momentum  and  energy  according  to  the  laws  of 
elastic  collision.  The  nucleus  thereby  becomes  a  high-speed 
charged  particle  which  will  produce  ionization  in  an  ionization 
chamber,  counter,  or  cloud  chamber.  But  if  the  neutron  has  low 
speed,  e.g.,  thermal,  the  struck  nucleus  will  not  get  enough 
energy  to  cause  ionization.  If,  on  the  other  hand,  the  neutron  is 
absorbed  and  the  resultant  nucleus  breaks  up  with  the  release  of 
energy,  ionization  will  be  produced.  Thus,  for  the  detection  of 
high-speed  neutrons  one  has  a  choice  between  elastic  collisions 
and  nuclear  reaction,  but  for  thermal  speeds  only  nuclear  reaction 
will  serve. 

The  reaction  most  commonly  used  is  the  <>B10(n,  a)sLi7  reaction 
which  releases  about  2.5  Mev  energy  shared  between  the  resultant 

Observing  Fast  Particles  233 

alpha  particle  and  aLi7  nucleus.  This  is  ample  to  produce  ioniza- 
tion.  This  reaction  is  used  by  filling  an  ionization  chamber  or 
proportional  counter  with  boron  trifluoride  gas  so  that  the  reac- 
tion occurs  in  the  region  where  ionization  is  wanted;  as  an  alterna- 
tive the  interior  of  the  chamber  or  counter  is  lined  with  boron. 
The  ionization  chamber  then  serves  as  an  instrument  to  measure 
overall  neutron  flux  while  the  proportional  counter  records 
numbers  of  individual  neutrons. 

One  of  the  most  valuable  methods  of  measuring  neutron  densi- 
ties by  nuclear  reactions  depends  on  the  production  of  artificial 
radioactive  nuclei.  A  foil  known  to  be  made  radioactive  by  neu- 
tron bombardment  is  inserted  at  a  point  where  the  neutron 
intensity  is  wanted.  After  a  given  time  it  is  removed  and  its 
activity  measured  by  an  electroscope  or  counter.  The  degree  of 
activity  that  has  been  built  up  is  then  a  measure  of  the  number 
of  neutrons  that  have  been  absorbed.  This  method  has  the 
obvious  disadvantage  that  it  does  not  give  an  instantaneous 
response  as  do  the  ionization  chamber  and  counter. 

One  of  the  most  interesting  methods  developed  on  the  project 
is  to  use  the  fission  of  uranium  as  the  nuclear  reaction  for  neutron 
detection.  Furthermore,  by  separating  the  isotopes,  fast  and  slow 
neutrons  can  be  differentiated. 

Since  the  probability  of  a  neutron  reaction  occurring  is  dif- 
ferent for  every  reaction  and  for  every  neutron  speed,  difficulties 
of  translating  counts  or  current  measurements  into  numbers  and 
speeds  of  neutrons  present  are  even  greater  than  for  other  nuclear 
particles.  No  one  need  be  surprised  if  two  able  investigators  give 
different  numbers  for  supposedly  the  same  nuclear  consant.  It 
is  only  by  an  intricate  series  of  interlocking  experiments  carefully 
compared  and  interpreted  that  the  fundamental  facts  can  be 
untangled  from  experimental  and  instrumental  variables. 



Since  the  proton  and  the  neutron  are  the  fundamental  particles 
out  of  which  all  nuclei  are  built,  it  would  seem  natural  to  use  the 
mass  of  one  or  the  other  of  them  as  a  unit  of  mass.  The  choice 
would  probably  be  the  proton,  which  is  the  nucleus  of  a  hydrogen 
atom.  There  are  good  reasons,  historical  and  otherwise,  why 
neither  the  proton  nor  the  neutron  was  chosen.  Instead,  the  mass 
unit  used  in  atomic  and  nuclear  physics  is  one  sixteenth  of  the 
mass  of  the  predominant  oxygen  isotope,  O16,  and  is  equal  to 
1.6603  X  10~24  gram.  Expressed  in  terms  of  this  unit,  the  mass 
of  the  proton  is  1.00758  and  the  mass  of  the  neutron  is  1.00893. 
(Chemists  usually  use  a  very  slightly  different  unit  of  mass.) 


The  unit  of  electric  charge  used  in  nuclear  science  is  the  posi- 
tive charge  of  the  proton.  It  is  equal  in  magnitude  but  opposite 
in  sign  to  the  charge  on  the  electron  and  is  therefore  often  called 
the  electronic  charge.  One  electronic  charge  is  1.60  X  10~19 
coulomb.  It  may  be  recalled  that  a  current  of  one  ampere  flowing 
for  one  second  conveys  a  charge  of  one  coulomb;  i.e.,  one  elec- 
tronic charge  equals  1.60  X  10~19  ampere  second. 


The  energy  unit  used  in  nuclear  physics  is  the  electron  volt, 
which  is  defined  as  equal  to  the  kinetic  energy  which  a  particle 
carrying  one  electronic  charge  acquires  in  falling  freely  through 
a  potential  drop  of  one  volt.  It  is  often  convenient  to  use  the 
million-times  greater  unit:  million  electron  volt  (Mev). 


Units  of  Mass,  Charge,  Energy 


The  relationships  among  the  electron  volt  and  other  common 
units  of  energy  are  in  the  following  table: 





g.  cal. 

kw.  hrs. 


1.07  X  ID"3 
1.60  X  10~6 
3.83  X  10~" 
4.45  X  10-20 

9.31  X  102 
1.49  X  10~3 
3.56  X  10~" 

4.15  X  10~17 

6.71  X  102 

6.24  X  105 
2.39  X  10~8 
2.78  X  10-" 

2.81  X  1010 
2.62  X  1013 
4.18  X  107 

1.16  X  10~6 

2.41  X  1016 

2.25  X  1019 
3.60  X  1013 
8.60  X  105 


mass  units 
g.  cal. 
kw.  hrs. 

g.  cal. 
kw.  hrs. 

mass  units 
g.  cal. 
kw.  hrs. 

mass  units 
kw.  hrs. 

mass  units 
g.  cal. 


As  was  pointed  out  in  Chapter  VI,  the  control  of  a  chain- 
reacting  pile  is  greatly  facilitated  by  the  fact  that  some  of  the 
neutrons  resulting  from  uranium  fission  are  not  emitted  until 
more  than  a  second  after  fission  occurs.  It  was  therefore  important 
to  study  this  effect  experimentally.  Such  experiments  were 
described  by  Snell,  Nedzel  and  Ibser  in  a  report  dated  May  15, 
1942  from  which  we  quote  as  follows: 

"The  present  experiment  consists  of  two  interrelated  parts — 
one  concerned  with  the  decay  curve,  and  one  concerned  with  the 
intensity  of  the  delayed  neutrons  measured  in  terms  of  that  of  the 
'instantaneous'  fission  neutrons. 


"The  neutron  source  was  the  beryllium  target  of  the  University 
of  Chicago  cyclotron  struck  by  a  beam  of  up  to  20  pA  of  8  Mev 
deuterons.  Near  the  target  was  placed  a  hollow  shell  made  of 
tinned  iron  and  containing  106  Ibs.  of  UaOg.  This  was  surrounded 
by  about  2"  of  paraffin.  The  interior  of  the  shell  was  filled  with 
paraffin,  except  for  an  axial  hole  which  accommodated  a  BF3- 
filled  proportional  counter.  The  counter  was  connected  through 
an  amplifier  to  a  scaling  circuit  ('scale  of  64')  equipped  with  inter- 
polating lights  and  a  Cenco  impulse  counter.  A  tenth-second 
timer,  driven  by  a  synchronous  motor,  and  hundredth-second  stop 
watch  were  mounted  on  the  panel  of  the  sealer,  close  to  the 
interpolating  lights  and  impulse  counter.  This  group  of  dials  and 
lights  was  photographed  at  an  appropriately  varying  rate  by  a 
Sept  camera  which  was  actuated  by  hand.  The  result  was  a 
record  on  movie  film  of  times  and  counts,  from  which  the  decay 
curves  were  plotted. 


Delayed  Neutrons  from  Uranium  Fission          237 

"The  actual  procedure  was  as  follows:  During  bombardment 
the  stop  watch  was  started  and  the  timer  was  running  con- 
tinuously; the  counter  and  amplifier  were  on,  but  the  pulses 
leaving  the  amplifier  were  grounded.  The  sealer  was  set  at  zero. 
After  a  warning  signal  the  cyclotron  was  shut  off  by  one  operator, 
while  another  operator  switched  the  output  of  the  amplifier  from 
ground  into  the  sealer,  and  started  taking  photographs.  It  was 
easy  to  take  the  first  photograph  within  half  a  second  of  turning 
off  the  cyclotron.  Sixty  to  a  hundred  photographs  were  taken 
during  a  typical  run.  The  necessity  of  using  both  a  stop  watch 
and  a  timer  arose  from  the  fact  that  the  hundredth-second  pre- 
cision of  the  stop  watch  was  needed  for  the  small  time  intervals 
between  photographs  during  the  initial  part  of  the  run,  but  the 
watch  ran  down  and  stopped  before  the  counting  was  complete. 
The  timer  then  gave  sufficient  precision  for  the  later  time 

"Some  forty  runs  were  taken  under  varying  experimental  con- 
ditions. Short  activations  of  one  or  two  seconds  were  given  for 
best  resolution  of  the  short  periods.  Long,  intense  bombardments 
lasting  15-20  minutes,  as  close  as  possible  to  the  target,  were 
made  to  make  the  long  period  activities  show  up  with  a  maximum 
intensity.  Some  5-minute  bombardments  were  made,  keeping  the 
cyclotron  beam  as  steady  as  possible,  to  study  the  relative  satura- 
tion intensities  of  the  various  activities;  in  these  activations  the 
cyclotron  beam  was  reduced  to  1  or  2  /xA  to  prevent  the  initial 
counting  rate  from  becoming  too  high  for  a  counter  (300  per  sec. 
\\  as  taken  as  a  reasonable  upper  limit  for  reliable  counting).  Two 
BF3  counters  were  available,  one  having  a  thermal  neutron  cross 
section  of  2.66  sq.  cm.,  and  the  other  0.43  sq.  cm.  After  a  strong 
activation,  we  could  follow  the  decay  of  the  delayed  neutrons 
for  some  13  minutes.  Background  counts  (presumably  chiefly 
due  to  spontaneous  fission  neutrons)  were  taken  and  were  sub- 
tracted from  the  readings.  They  amounted  to  about  0.4  counts 
per  sec.  for  the  large  counter. 

"A  study  of  all  the  decay  curves  gives  the  following  as  a  general 
picture  of  the  neutron-emitting  activities  present: 

238  Appendix  3 



57  ±  3  sec.  0.135 

24  ±  2  sec.  1.0 

7          sec.  1.2 

2.5       sec.  1.2 

"Any  activity  of  period  longer  than  57  sec.  failed  to  appear 
even  after  the  most  intense  bombardment  we  could  give,  lasting 
20  minutes.  The  relative  initial  intensities  given  are  the  average 
values  obtained  from  three  curves. 

"These  results  give  the  following  equation  for  the  decay  curve, 
of  the  delayed  neutrons  after  activation  to  saturation: 

Activity  =  constant  (1.2e-°-28<  +  1.2e-°-099'  +  1.0e-°-029< 

-f  0.135e-°-0120 
where  t  is  in  seconds." 

The  second  part  of  the  experiment  measured  the  total  number 
of  neutrons  emitted  in  the  time  interval  0.01  sec.  to  2.0  min.  after 
the  cyclotron  was  turned  off.  Assuming  that  all  the  delayed 
neutrons  observed  were  in  the  four  groups  measured  in  the  first 
part  of  the  experiment,  this  second  result  indicated  that  1.0  ±  0.2 
per  cent  of  the  neutrons  emitted  in  uranium  fission  are  delayed 
by  at  least  0.01  sec.  and  that  about  0.07  per  cent  are  delayed  by 
as  much  as  a  minute.  By  designing  the  effective  value  of  k,  the 
multiplication  factor,  for  a  typical  operating  pile  to  be  only  1.01 
with  all  the  controls  removed  and  the  total  variation  in  k  from 
one  control  rod  to  be  0.002,  the  number  of  delayed  neutrons  is 
sufficient  to  allow  easy  control. 


In  Chapter  VI  the  construction  and  operation  of  the  first  self- 
sustaining  chain-reacting  pile  were  described  briefly.  Though 
details  must  still  be  withheld  for  security  reasons,  the  following 
paragraphs  give  a  somewhat  fuller  description  based  on  a  report 
by  Fermi.  This  pile  was  erected  by  Fermi  and  his  collaborators  in 
the  fall  of  1942. 


The  original  plan  called  for  an  approximately  spherical  pile 
with  the  best  materials  near  the  center.  Actually  control  measure- 
ments showed  that  the  critical  size  had  been  reached  before  the 
sphere  was  complete,  and  the  construction  was  modified  accord- 
ingly. The  final  structure  may  be  roughly  described  as  an  oblate 
spheroid  flattened  at  the  top,  i.e.,  like  a  door  knob.  It  was  desired 
to  have  the  uranium  or  uranium  oxide  lumps  spaced  in  a  cubic 
lattice  imbedded  in  graphite.  Consequently,  the  graphite  was 
cut  in  bricks  and  built  up  in  layers,  alternate  ones  of  which  con- 
tained lumps  of  uranium  at  the  corners  of  squares.  The  critical 
size  was  reached  when  the  pile  had  been  built  to  a  height  only 
three  quarters  of  that  needed  according  to  the  most  cautious 
estimates.  Consequently  only  one  more  layer  was  added.  The 
graphite  used  was  chiefly  from  the  National  Carbon  Company 
and  the  Speer  Graphite  Company.  The  pile  contained  12,400  Ibs. 
of  metal,  part  of  which  was  supplied  by  Westinghouse,  part  by 
Metal  Hydrides,  and  part  by  Ames.  Since  there  were  many  more 
lattice  points  than  lumps  of  metal,  the  remaining  ones  were  filled 
with  pressed  oxide  lumps. 

For  purposes  of  control  and  experiment  there  were  ten  slots 
passing  completely  through  the  pile.  Three  of  those  near  the 


240  Appendix  4 

center  were  used  for  control  and  safety  rods.  Further  to  facilitate 
experiment,  particularly  the  removal  of  samples,  one  row  of 
graphite  bricks  carrying  uranium  and  passing  near  the  center  of 
the  pile  was  arranged  so  that  it  could  be  pushed  completely  out 
of  the  pile. 

This  whole  graphite  sphere  was  supported  by  a  timber  frame- 
work resting  on  the  floor  of  a  squash  cpurt  under  the  West  Stands 
of  Stagg  Field. 


The  metal  lattice  at  the  center  of  the  pile  and  the  two  other 
major  lattices  making  up  the  bulk  of  the  rest  of  the  pile  had  each 
been  studied  separately  in  exponential  experiments  #18,  #27,  and 
#29.  These  had  given  a  multiplication  factor  of  1.07  for  the  metal 
lattice  and  1.04  and  1.03  for  the  oxide  lattices,  the  difference  in 
the  last  two  resulting  from  difference  in  the  grade  of  graphite  used. 
It  is  to  be  remembered  that  these  figures  are  multiplication  factors 
for  lattices  of  infinite  size.  Therefore  a  prediction  of  the  actual 
effective  multiplication  factor  keff  for  the  pile  as  constructed 
depended  on  the  validity  of  the  deduction  of  k  from  the  ex- 
ponential experiments,  on  a  proper  averaging  for  the  different 
lattices,  and  on  a  proper  deduction  of  keff  from  the  average  k  for 
infinite  size.  Although  the  original  design  of  the  pile  had  been 
deliberately  generous,  its  success  when  only  partly  completed 
indicated  that  the  values  of  the  multiplication  factors  as  calculated 
from  exponential  experiments  had  been  too  low.  The  observed 
effective  multiplication  factor  of  the  part  of  the  planned  structure 
actually  built  was  about  1 .0006  when  all  neutron  absorbers  were 


A  series  of  measurements  was  made  while  the  pile  was  being 
assembled  in  order  to  be  sure  that  the  critical  dimensions  were 
not  reached  inadvertently.  These  measurements  served  also  to 
check  the  neutron  multiplication  properties  of  the  structure 

First  Self-Sustaining  Pile 


during   assembly,   making   possible   a   prediction   of  where   the 
critical  point  would  be  reached. 

In  general,  any  detector  of  neutrons  or  gamma  radiation  can 
be  used  for  measuring  the  intensity  of  the  reaction.  Neutron 
detectors  are  somewhat  preferable  since  they  give  response  more 





quickly  and  are  not  affected  by  fission-product  radiations  after 
shut  down.  Actually  both  neutron  detectors  (boron  trifluoride 
counters)  and  gamma-ray  ionization  chambers  were  distributed 
in  and  around  the  pile.  Certain  of  the  ionization  chambers  were 
used  to  operate  recording  instruments  and  automatic  safety 

In  the  pile  itself  measurements  were  made  with  two  types  of 
detector.  A  boron  trifluoride  counter  was  inserted  in  a  slot  about 

242  Appendix  4 

43"  from  the  ground  and  its  readings  taken  at  frequent  intervals. 
In  addition,  an  indium  foil  was  irradiated  every  night  in  a  posi- 
tion as  close  as  possible  to  the  effective  center  of  the  pile,  and  its 
induced  activity  was  measured  the  following  morning  and  com- 
pared with  the  readings  of  the  boron  trifluoride  counter. 

The  results  of  such  measurements  can  be  expressed  in  two  ways. 
Since  the  number  of  secondary  neutrons  produced  by  fission  will 
increase  steadily  as  the  pile  is  constructed,  the  activity  A  induced 
in  a  standard  indium  foil  at  the  center  will  increase  steadily  as 
the  number  of  layers  of  the  pile  is  increased.  Once  the  effective 
multiplication  factor  is  above  one,  A  would  theoretically  increase 
to  infinity.  Such  an  approach  to  infinity  is  hard  to  observe,  so  a 
second  way  of  expressing  the  results  was  used.  Suppose  the  lattice 
spacing  and  purity  of  materials  of  a  graphite-uranium  structure 
are  such  that  the  multiplication  factor  would  be  exactly  one  if 
the  structure  were  a  sphere  of  infinite  radius.  Then,  for  an  actual 
sphere  of  similar  construction  but  finite  radius,  the  activation  of  a 
detector  placed  at  the  center  would  be  proportional  to  the  square 
of  the  radius.  It  was  possible  to  determine  a  corresponding 
effective  radius  R«ff  for  the  real  pile  in  each  of  its  various  stages. 
It  followed,  therefore,  that,  if  the  factor  k^  were  precisely  one  on 
the  average  for  the  lattice  in  the  pile,  the  activity  A  of  the  de- 
tector at  the  center  should  increase  with  increasing  Reff  in  such 
a  way  that  R2eff/A  remained  constant,  but,  if  k*,  for  the  lattice 
were  greater  than  one,  then  as  the  pile  size  approached  the 
critical  value,  that  is,  as  k^ff  approached  one,  A  should  approach 
infinity  and  therefore  R2eff/A  approach  zero.  Therefore  by  extra- 
polating a  curve  of  R2eff/A  vs.  size  of  the  pile  i.e.,  number  of  layers, 
to  where  it  cut  the  axis,  it  was  possible  to  predict  at  what  layer 
keff  would  become  one.  Such  a  curve,  shown  in  Fig.  1,  indicated 
at  what  layer  the  critical  size  would  be  reached.  The  less  useful 
but  more  direct  and  dramatic  way  of  recording  the  results  is 
shown  in  Fig.  2,  which  shows  the  growth  of  the  neutron  activity 
of  the  pile  as  layers  were  added. 

During  the  construction,  appreciably  before  reaching  this 
critical  layer,  some  cadmium  strips  were  inserted  in  suitable  slots. 

First  Self -Sustaining  Pile 









FIGURE     2 

244  Appendix  4 

They  were  removed  once  every  day  with  the  proper  precautions 
in  order  to  check  the  approach  to  the  critical  conditions.  The 
construction  was  carried  in  this  way  to  the  critical  layer. 


The  reaction  was  controlled  by  inserting  in  the  pile  some  strips 
of  neutron-absorbing  material — cadmium  or  boron  steel.  When 
the  pile  was  not  in  operation,  several  such  cadmium  strips  were 
inserted  in  a  number  of  slots,  bringing  the  effective  multiplication 
factor  considerably  below  one.  In  fact,  any  one  of  the  cadmium 
strips  alone  was  sufficient  to  bring  the  pile  below  the  critical 
condition.  Besides  cadmium  strips  that  could  be  used  for  manual 
operation  of  the  pile,  two  safety  rods  and  one  automatic  control 
rod  were  provided.  The  automatic  control  rod  was  operated  by 
two  electric  motors  responding  to  an  ionization  chamber  and 
amplifying  system  so  that,  if  the  intensity  of  the  reaction  increased 
above  the  desired  level,  the  rod  was  pushed  in,  and  vice  versa. 


To  operate  the  pile  all  but  one  of  the  cadmium  strips  were 
taken  out.  The  remaining  one  was  then  slowly  pulled  out.  As 
the  critical  conditions  were  approached,  the  intensity  of  the 
neutrons  emitted  by  the  pile  began  to  increase  rapidly.  It  should 
be  noticed,  however,  that,  when  this  last  strip  of  cadmium  was  so 
far  inside  the  pile  that  the  effective  multiplication  factor  was  just 
below  one,  it  took  a  rather  long  time  for  the  intensity  to  reach 
the  saturation  value.  Similarly,  if  the  cadmium  strip  was  just 
far  enough  out  to  make  keff  greater  than  one,  the  intensity  rose 
at  a  rather  slow  rate.  For  example,  if  one  rod  is  only  1  cm.  out 
from  the  critical  position,  the  "relaxation  time,"  i.e.,  the  time 
for  the  intensity  to  double,  is  about  four  hours.  These  long 
"relaxation  times"  were  the  result  of  the  small  percentage  of 
delayed  neutrons  which  have  been  discussed  in  Appendix  3,  and 
make  it  relatively  easy  to  keep  the  pile  operating  at  a  constant 
level  of  intensity. 

First  Self-sustaining  Pile  245 

The  pile  was  first  operated  on  December  2,  1 942  to  a  maximum 
energy  production  of  about  J^  watt.  On  December  12th  the 
intensity  was  run  up  to  about  200  watts,  but  it  was  not  felt  safe 
to  go  higher  because  of  the  danger  of  the  radiation  to  personnel 
in  and  around  the  building.  During  this  high  intensity  run, 
measurements  were  made  of  radiation  intensity  beside  the  pile, 
in  the  building,  and  on  the  sidewalk  outside. 


Presented  below  is  a  list  of  titles  of  representative  reports  pre- 
pared in  the  Metallurgical  Laboratory  of  the  University  of  Chi- 
cago in  1942. 

A  Table  for  Calculating  the  Percentage  Loss  Due  to  the 

Presence  of  Impurities  in  Alloy 

Concerning  the  Radium-Beryllium  Neutron  Sources 
Preliminary    Estimates    of    the    Radiations    from    Fission 


Background  of  Natural  Neutrons  in  Multiplying  Pile 
Absorption  Cross  Sections  for  Rn  plus  Be  Fast  Neutrons 
On   Mechanical   Stresses   Produced   by  Temperature  Gra- 
dients in  Rods  and  Spheres 

Effect  of  Geometry  on  Resonance  Absorption  of  Neutrons  by 

Protection  against  Radiations 

Planning  Experiments  on  Liquid  Cooling 

Report  on  the  Possibility  of  Purifying  Uranium  by  Carbonyl 

Formation  and  Decomposition 
On  the  Radioactivity  of  Cooling  Helium 
Estimation  of  Stability  of  Ether  under  Various  Conditions  of 

Uranium  Poisoning 

Transuranic  and  Fission  Product  Activities 
Chemical  Effects  of  Radiation  on  Air  Surrounding  the  Pile 
An  Estimate  of  the  Chemical  Effects  of  Radiation  on  the 

Cooling  Water  in  the  Pile 

The  Extraction  Method  of  Purification  of  Uranyl  Nitrate 
The  Diffusion  of  Fission  Products  from  Cast  Metal  at  600°C 

and  1000°C 


ON  NEW  MEXICO  TEST,  JULY  16,  1945 

"Mankind's  successful  transition  to  a  new  age,  the  Atomic  Age, 
was  ushered  in  July  16,  1945,  before  the  eyes  of  a  tense  group  of 
renowned  scientists  and  military  men  gathered  in  the  desertlands 
of  New  Mexico  to  witness  the  first  end  results  of  their  $2,000,000,- 
000  effort.  Here  in  a  remote  section  of  the  Alamogordo  Air  Base 
120  miles  southeast  of  Albuquerque  the  first  man-made  atomic 
explosion,  the  outstanding  achievement  of  nuclear  science,  was 
achieved  at  5:  30  a.m.  of  that  day.  Darkening  heavens,  pouring 
forth  ram  and  lightning  immediately  up  to  the  zero  hour,  height- 
ened the  drama. 

"Mounted  on  a  steel  tower,  a  revolutionary  weapon  destined 
to  change  war  as  we  know  it,  or  which  may  even  be  the  instru- 
mentality to  end  all  wars,  was  set  off  with  an  impact  which 
signalized  man's  entrance  into  a  new  physical  world.  Success  was 
greater  than  the  most  ambitious  estimates.  A  small  amount  of 
matter,  the  product  of  a  chain  of  huge  specially  constructed 
industrial  plants,  was  made  to  release  the  energy  of  the  universe 
locked  up  within  the  atom  from  the  beginning  of  time.  A  fabulous 
achievement  had  been  reached.  Speculative  theory,  barely  estab- 
lished in  pre-war  laboratories,  had  been  projected  into  practicality. 

"This  phase  of  the  Atomic  Bomb  Project,  which  is  headed  by 
Major  General  Leslie  R.  Groves,  was  under  the  direction  of  Dr. 
J.  R.  Oppenheimer,  theoretical  physicist  of  the  University  of 
California.  He  is  to  be  credited  with  achieving  the  implementa- 
tion of  atomic  energy  for  military  purposes. 

"Tension  before  the  actual  detonation  was  at  a  tremendous 
pitch.  Failure  was  an  ever-present  possibility.  Too  great  a  success, 
envisioned  by  some  of  those  present,  might  have  meant  an  uncon- 
trollable, unusable  weapon. 

"Final  assembly  of  the  atomic  bomb  began  on  the  night  of 


248  Appendix  6 

July  12  in  an  old  ranch  house.  As  various  component  assemblies 
arrived  from  distant  points,  tension  among  the  scientists  rose  to 
an  increasing  pitch.  Coolest  of  all  was  the  man  charged  with  the 
actual  assembly  of  the  vital  core,  Dr.  R.  F.  Bacher,  in  normal 
times  a  professor  at  Cornell  University. 

"The  entire  cost  of  the  project,  representing  the  erection  of 
whole  cities  and  radically  new  plants  spread  over  many  miles  of 
countryside,  plus  unprecedented  experimentation,  was  repre- 
sented in  the  pilot  bomb  and  its  parts.  Here  was  the  focal  point 
of  the  venture.  No  other  country  in  the  world  had  been  capable 
of  such  an  outlay  in  brains  and  technical  effort. 

"The  full  significance  of  these  closing  moments  before  the  final 
factual  test  was  not  lost  on  these  men  of  science.  They  fully  knew 
their  position  as  pioneers  into  another  age.  They  also  knew  that 
one  false  move  would  blast  them  and  their  entire  effort  into 
eternity.  Before  the  assembly  started  a  receipt  for  the  vital  matter 
was  signed  by  Brigadier  General  Thomas  F.  Farrell,  General 
Groves'  deputy.  This  signalized  the  formal  transfer  of  the  irre- 
placeable material  from  the  scientists  to  the  Army. 

"During  final  preliminary  assembly,  a  bad  few  minutes  devel- 
oped when  the  assembly  of  an  important  section  of  the  bomb  was 
delayed.  The  entire  unit  was  machine- tooled  to  the  finest  meas- 
urement. The  insertion  was  partially  completed  when  it  appar- 
ently wedged  tightly  and  would  go  no  farther.  Dr.  Bacher, 
however,  was  undismayed  and  reassured  the  group  that  time 
would  solve  the  problem.  In  three  minutes'  time,  Dr.  Bacher's 
statement  was  verified  and  basic  assembly  was  completed  without 
further  incident. 

"Specialty  teams,  comprised  of  the  top  men  on  specific  phases 
of  science,  all  of  which  were  bound  up  in  the  whole,  took  over 
their  specialized  parts  of  the  assembly.  In  each  group  was  central- 
ized months  and  even  years  of  channelized  endeavor. 

"On  Saturday,  July  14,  the  unit  which  was  to  determine  the 
success  or  failure  of  the  entire  project  was  elevated  to  the  top  of 
the  steel  tower.  All  that  day  and  the  next,  the  job  of  preparation 
went  on.  In  addition  to  the  apparatus  necessary  to  cause  the 

New  Mexico  Test,  July  16,  1945  249 

detonation,  complete  instrumentation  to  determine  the  pulse 
beat  and  all  reactions  of  the  bomb  was  rigged  on  the  tower. 

"The  ominous  weather  which  had  dogged  the  assembly  of  the 
bomb  had  a  very  sobering  affect  on  the  assembled  experts  whose 
work  was  accomplished  amid  lightning  flashes  and  peals  of 
thunder.  The  weather,  unusual  and  upsetting,  blocked  out  aerial 
observation  of  the  test.  It  even  held  up  the  actual  explosion 
scheduled  at  4:00  a.m.  for  an  hour  and  a  half.  For  many  months 
the  approximate  date  and  time  had  been  set  and  had  been  one 
of  the  high-level  secrets  of  the  best  kept  secret  of  the  entire  war. 

"Nearest  observation  point  was  set  up  10,000  yards  south  of 
the  tower  where  in  a  timber  and  earth  shelter  the  controls  for  the 
test  were  located.  At  a  point  17,000  yards  from  the  tower  at  a 
point  which  would  give  the  best  observation  the  key  figures  in 
the  atomic  bomb  project  took  their  posts.  These  included  General 
Groves,  Dr.  Vannevar  Bush,  head  of  the  Office  of  Scientific 
Research  and  Development  and  Dr.  James  B.  Conant,  president 
of  Harvard  University. 

"Actual  detonation  was  in  charge  of  Dr.  K.  T.  Bainbridge  of 
Massachusetts  Institute  of  Technology.  He  and  Lieutenant  Bush, 
in  charge  of  the  Military  Police  Detachment,  were  the  last  men 
to  inspect  the  tower  with  its  cosmic  bomb. 

"At  three  o'clock  in  the  morning  the  party  moved  forward  to 
the  control  station.  General  Groves  and  Dr.  Oppenheimer  con- 
sulted with  the  weathermen.  The  decision  was  made  to  go  ahead 
with  the  test  despite  the  lack  of  assurance  of  favorable  weather. 
The  time  was  set  for  5:30  a.m. 

"General  Groves  rejoined  Dr.  Conant  and  Dr.  Bush,  and  just 
before  the  test  time  they  joined  the  many  scientists  gathered  at 
the  Base  Camp.  Here  all  present  were  ordered  to  lie  on  the 
ground,  face  downward,  heads  away  from  the  blast  direction. 

"Tension  reached  a  tremendous  pitch  in  the  control  room  as 
the  deadline  approached.  The  several  observation  points  in  the 
area  were  tied  in  to  the  control  room  by  radio  and  with  twenty 
minutes  to  go,  Dr.  S.  K.  Allison  of  Chicago  University  took  over 
the  radio  net  and  made  periodic  time  announcements. 

250  Appendix  6 

"The  time  signals,  'minus  20  minutes,  minus  fifteen  minutes,' 
and  on  and  on  increased  the  tension  to  the  breaking  point  as  the 
group  in  the  control  room  which  included  Dr.  Oppenheimer  and 
General  Farrell  held  their  breaths,  all  praying  with  the  intensity 
of  the  moment  which  will  live  forever  with  each  man  who  was 
there.  At  'minus  45  seconds/  robot  mechanism  took  over  and 
from  that  point  on  the  whole  great  complicated  mass  of  intricate 
mechanism  was  in  operation  without  human  control.  Stationed 
at  a  reserve  switch,  however,  was  a  soldier  scientist  ready  to 
attempt  to  stop  the  explosion  should  the  order  be  issued.  The 
order  never  came. 

"At  the  appointed  time  there  was  a  blinding  flash  lighting  up 
the  whole  area  brighter  than  the  brightest  daylight.  A  mountain 
range  three  miles  from  the  observation  point  stood  out  in  bold 
relief.  Then  came  a  tremendous  sustained  roar  and  a  heavy 
pressure  wave  which  knocked  down  two  men  outside  the  control 
center.  Immediately  thereafter,  a  huge  multi-colored  surging 
cloud  boiled  to  an  altitude  of  over  40,000  feet.  Clouds  in  its  path 
disappeared.  Soon  the  shifting  substratosphere  winds  dispersed 
the  now  grey  mass. 

"The  test  was  over,  the  project  a  success. 

"The  steel  tower  had  been  entirely  vaporized.  Where  the  tower 
had  stood,  there  was  a  huge  sloping  crater.  Dazed  but  relieved  at 
the  success  of  their  tests,  the  scientists  promptly  marshalled  their 
forces  to  estimate  the  strength  of  America's  new  weapon.  To 
examine  the  nature  of  the  crater,  specially  equipped  tanks  were 
wheeled  into  the  area,  one  of  which  carried  Dr.  Enrico  Fermi, 
noted  nuclear  scientist.  Answer  to  their  findings  rests  in  the 
destruction  effected  in  Japan  today  in  the  first  military  use  of  the 
atomic  bomb. 

"Had  it  not  been  for  the  desolated  area  where  the  test  was  held 
and  for  the  cooperation  of  the  press  in  the  area,  it  is  certain  that 
the  test  itself  would  have  attracted  far-reaching  attention.  As  it 
was,  many  people  in  that  area  are  still  discussing  the  effect  of  the 
smash.  A  significant  aspect,  recorded  by  the  press,  was  the  experi- 
ence of  a  blind  girl  near  Albuquerque  many  miles  from  the 

New  Mexico  Test,  July  76,  7945  251 

scene,  who,  when  the  flash  of  the  test  lighted  the  sky  before  the 
explosion  could  be  heard,  exclaimed,  'What  was  that?' 

"Interviews  of  General  Groves  and  General  Farrell  give  the 
following  on-the-scene  versions  of  the  test.  General  Groves  said: 
'My  impressions  of  the  night's  high  points  follow:  After  about  an 
hour's  sleep  I  got  up  at  0100  and  from  that  time  on  until  about 
five  I  was  with  Dr.  Oppenheimer  constantly.  Naturally  he  was 
tense,  although  his  mind  was  working  at  its  usual  extraordinary 
efficiency.  I  attempted  to  shield  him  from  the  evident  concern 
shown  by  many  of  his  assistants  who  were  disturbed  by  the  un- 
certain weather  conditions.  By  0330  we  decided  that  we  could 
probably  fire  at  0530.  By  0400  the  rain  had  stopped  but  the  sky 
was  heavily  overcast.  Our  decision  became  firmer  as  time 
went  on. 

"  '  During  most  of  these  hours  the  two  of  us  journeyed  from  the 
control  house  out  into  the  darkness  to  look  at  the  stars  and  to 
assure  each  other  that  the  one  or  two  visible  stars  were  becoming 
brighter.  At  0510  I  left  Dr.  Oppenheimer  and  returned  to  the 
main  observation  point  which  was  17,000  yards  from  the  point 
of  explosion.  In  accordance  with  our  orders  I  found  all  personnel 
not  otherwise  occupied  massed  on  a  bit  of  high  ground. 

"  'Two  minutes  before  the  scheduled  firing  time,  all  persons  lay 
face  down  with  their  feet  pointing  towards  the  explosion.  As  the 
remaining  time  was  called  from  the  loud  speaker  from  the  10,000- 
yard  control  station  there  was  complete  awesome  silence.  Dr. 
Conant  said  he  had  never  imagined  seconds  could  be  so  long. 
Most  of  the  individuals  in  accordance  with  orders  shielded  their 
eyes  in  one  way  or  another. 

"  'First  came  the  burst  of  light  of  a  brilliance  beyond  any  com- 
parison. We  all  rolled  over  and  looked  through  dark  glasses  at 
the  ball  of  fire.  About  forty  seconds  later  came  the  shock  wave 
followed  by  the  sound,  neither  of  which  seemed  startling  after 
our  complete  astonishment  at  the  extraordinary  lighting  intensity. 

"  'A  massive  cloud  was  formed  which  surged  and  billowed 
upward  with  tremendous  power,  reaching  the  substratosphere  in 
about  five  minutes. 

252  Appendix  6 

"  'Two  supplementary  explosions  of  minor  effect  other  than  the 
lighting  occurred  in  the  cloud  shortly  after  the  main  explosion. 

"  'The  cloud  traveled  to  a  great  height  first  in  the  form  of  a 
ball,  then  mushroomed,  then  changed  into  a  long  trailing  chim- 
ney-shaped column  and  finally  was  sent  in  several  directions  by 
the  variable  winds  at  the  different  elevations. 

"  'Dr.  Conant  reached  over  and  we  shook  hands  in  mutual  con- 
gratulations. Dr.  Bush,  who  was  on  the  other  side  of  me,  did 
likewise.  The  feeling  of  the  entire  assembly,  even  the  uninitiated, 
was  of  profound  awe.  Drs.  Conant  and  Bush  and  myself  were 
struck  by  an  even  stronger  feeling  that  the  faith  of  those  who  had 
been  responsible  for  the  initiation  and  the  carrying  on  of  this 
Herculean  project  had  been  justified." 

General  FarrelPs  impressions  are:  "The  scene  inside  the  shelter 
was  dramatic  beyond  words.  In  and  around  the  shelter  were  some 
twenty  odd  people  concerned  with  last-minute  arrangements. 
Included  were  Dr.  Oppenheimer,  the  Director  who  had  borne 
the  great  scientific  burden  of  developing  the  weapon  from  the 
raw  materials  made  in  Tennessee  and  Washington,  and  a  dozen 
of  his  key  assistants,  Dr.  Kistiakowsky,  Dr.  Bainbridge,  who  super- 
vised all  the  detailed  arrangements  for  the  test;  the  weather 
expert,  and  several  others.  Besides  those,  there  were  a  handful 
of  soldiers,  two  or  three  Army  officers  and  one  Naval  Officer. 
The  shelter  was  filled  with  a  great  variety  of  instruments  and 

"  'For  some  hectic  two  hours  preceding  the  blast,  General 
Groves  stayed  with  the  Director.  Twenty  minutes  before  the 
zero  hour,  General  Groves  left  for  his  station  at  the  base  camp, 
first  because  it  provided  a  better  observation  point  and  second, 
because  of  our  rule  that  he  and  I  must  not  be  together  in  situa- 
tions where  there  is  an  element  of  danger  which  existed  at  both 

"  'Just  after  General  Groves  left,  announcements  began  to  be 
broadcast  of  the  interval  remaining  before  the  blast  to  the  other 
groups  participating  in  and  observing  the  test.  As  the  time  interval 
grew  smaller  and  changed  from  minutes  to  seconds,  the  tension 

New  Mexico  Test,  July  16,  1945  253 

increased  by  leaps  and  bounds.  Everyone  in  that  room  knew  the 
awful  potentialities  of  the  thing  that  they  thought  was  about  to 
happen.  The  scientists  felt  that  their  figuring  must  be  right  and 
that  the  bomb  had  to  go  off  but  there  was  in  everyone's  mind  a 
strong  measure  of  doubt. 

"  'We  were  reaching  into  the  unknown  and  we  did  not  know 
what  might  come  of  it.  It  can  safely  be  said  that  most  of  those 
present  were  praying — and  praying  harder  than  they  had  ever 
prayed  before.  If  the  shot  were  successful,  it  was  a  justification 
of  the  several  years  of  intensive  effort  of  tens  of  thousands  of 
people — statesmen,  scientists,  engineers,  manufacturers,  soldiers, 
and  many  others  in  every  walk  of  life. 

"'In  that  brief  instant  in  the  remote  New  Mexico  desert,  the 
tremendous  effort  of  the  brains  and  brawn  of  all  these  people 
came  suddenly  and  startlingly  to  the  fullest  fruition.  Dr.  Oppen- 
heimer,  on  whom  had  rested  a  very  heavy  burden,  grew  tenser 
as  the  last  seconds  ticked  off.  He  scarcely  breathed.  He  held  on 
to  a  post  to  steady  himself.  For  the  last  few  seconds,  he  stared 
directly  ahead  and  then  when  the  announcer  shouted  "Now!" 
and  there  came  this  tremendous  burst  of  light  followed  shortly 
thereafter  by  the  deep  growling  roar  of  the  explosion,  his  face 
relaxed  into  an  expression  of  tremendous  relief.  Several  of  the 
observers  standing  back  of  the  shelter  to  watch  the  lighting  effects 
were  knocked  flat  by  the  blast. 

"  'The  tension  in  the  room  let  up  and  all  started  congratulating 
each  other.  Everyone  sensed  'This  is  it!'.  No  matter  what  might 
happen  now  all  knew  that  the  impossible  scientific  job  had  been 
done.  Atomic  fission  would  no  longer  be  hidden  in  the  cloisters 
of  the  theoretical  physicists'  dreams.  It  was  almost  full  grown  at 
birth.  It  was  a  great  new  force  to  be  used  for  good  or  for  evil. 
There  was  a  feeling  in  that  shelter  that  those  concerned  with  its 
nativity  should  dedicate  their  lives  to  the  mission  that  it  would 
always  be  used  for  good  and  never  for  evil. 

"  'Dr.  Kistiakowsky  threw  his  arms  around  Dr.  Oppenheimer 
and  embraced  him  with  shouts  of  glee.  Others  were  equally 
enthusiastic.  All  the  pent-up  emotions  were  released  in  those  few 

254  Appendix  6 

minutes  and  all  seemed  to  sense  immediately  that  the  explosion 
had  far  exceeded  the  most  optimistic  expectations  and  wildest 
hopes  of  the  scientists.  All  seemed  to  feel  that  they  had  been 
present  at  the  birth  of  a  new  age — The  Age  of  Atomic  Energy — 
and  felt  their  profound  responsibility  to  help  in  guiding  into 
right  channels  the  tremendous  forces  which  had  been  unlocked 
for  the  first  time  in  history. 

"'As  to  the  present  war,  there  was  a  feeling  that  no  matter 
what  else  might  happen,  we  now  had  the  means  to  insure  its 
speedy  conclusion  and  save  thousands  of  American  lives.  As  to 
the  future,  there  had  been  brought  into  being  something  big 
and  something  new  that  would  prove  to  be  immeasurably  more 
important  than  the  discovery  of  electricity  or  any  of  the  other 
great  discoveries  which  have  so  affected  our  existence. 

"  'The  effects  could  well  be  called  unprecedented,  magnificent, 
beautiful,  stupendous  and  terrifying.  No  man-made  phenomenon 
of  such  tremendous  power  had  ever  occurred  before.  The  lighting 
effects  beggared  description.  The  whole  country  was  lighted  by  a 
searing  light  with  the  intensity  many  times  that  of  the  midday 
sun.  It  was  golden,  purple,  violet,  gray  and  blue.  It  lighted  every 
peak,  crevasse  and  ridge  of  the  nearby  mountain  range  with  a 
clarity  and  beauty  that  cannot  be  described  but  must  be  seen  to 
be  imagined.  It  was  that  beauty  the  great  poets  dream  about  but 
describe  most  poorly  and  inadequately.  Thirty  seconds  after,  the 
explosion  came  first,  the  air  blast  pressing  hard  against  the  people 
and  things,  to  be  followed  almost  immediately  by  the  strong, 
sustained,  awesome  roar  which  warned  of  doomsday  and  made 
us  feel  that  we  puny  things  were  blasphemous  to  dare  tamper  with 
the  forces  heretofore  reserved  to  the  Almighty.  Words  are  inade- 
quate tools  for  the  job  of  acquainting  those  not  present  with  the 
physical,  mental  and  psychological  effects.  It  had  to  be  witnessed 
to  be  realized.' " 



References  are  to  chapter  and  paragraph  except  references  to  appendices  which 
are  shown  as  A-7  etc. 

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Coryell,  C.  D.,  6:33 

Creutz,  E.  C.,  4:12,  6:36 

Crist,  R.  H.,  9:40 

Curie,  I.,  1:19,1:27 

Curie,  M.,  1:6 

Curie,  P.,  1:6 

Curme,  G.  O.,  Jr.,  5:5 

Currie,  L.  M.,  10:23 

Daniels,  F.,  7:48 
Dempster,  A.  J.,  7:48,  9:29 
Dickel,  G.,  9:22 
Doan,  R.  L.,  6:5,  6:8,  7:48 
Dunning,  J.  R.,  1:53,  10:1,  10:2, 
10:23,  10:41 

Eckart,  C.  H.,  3:14,  4:22 
Einstein,  A.,  1:4,  1:9,  1:39,  1:53, 


Eisenhart,  L.  P.,  3:3 
Emmett,  P.  H.,  10:23 
Enskog,  D.,  9:21 

Farrell,  Gen.  T.  F.,  5:32,  A-6 

Feld,  B.,  4:13 

Fermi,  E.,  1:28,  1:52,  2:1,  2:10, 
2:11,  3:1,  3:4,  3:6,  3:8,  3:14, 
4:2,  4:13,  4:17,  4:22,  5:12,  6:5, 
6:22,  6:31,  7:48,  8:27,  12:23, 
A-4,  A-6 

Feynman,  R.  P.,  9:32 
Finkelstein,  T.,  11:6 
Fletcher,  H.,  3:3 
Fowler,  R.  H.,  3:17,  4:38 
Franck,J.,  6:5,  7:48 
Fred,  E.  B.,  3:3 
Frisch,  O.  R.,  1:53,  1:54 

Gary,  T.  C.,  6:6 

Gherardi,  B.,  3:15 

Greenewalt,  C.  H.,  6:6,  7:50 

Grosse,  A.  von,  4:39,  9:40,  10:1, 

Groves,  Gen.  L.  R.,  5:24,  5:25, 
5:27,  5:28,  5:31,  5:32,  5:33, 
7:4,  7:5,  7:12,  8:50,  10:34, 
11:18,  12:2,  12:5,  13:5,  13:6, 

Gunn,  R.,  3:9,  4:36,  5:2 

Hahn,  O.,  1:53,  1:54 

Halban,  H.,  5:14 

Hamilton,  Dr.,  8:70 

Helmholz,  A.  C.,  11:6 

Henne,  A.  L.,  10:29 

Heydenburg,  N.  P.,  4:29 

Hertz,  G.,  9:15 

Hilberry,    N.,    6:8,    6:20,    6:33, 

6:45,  7:48 

Hoffman,  J.  I.,  6:12 
Hogness,  T.,  6:5,  7:48 
Hoover,  Comm.  G.  C.,  3:4,  3:5, 


Jenkins,  F.  A.,  11:6 
Jewett,  F.  B.,  3:15 
Johns,  I.  B.,  6:33 
Johnson,  C.  A.,  10:26 
Johnson,  W.  C.,  6:5,  7:48 
Joliot,  F.,  1:19,  1:27,  1:53,  3:2 

Kaplan,  L,  9:32 
Keith,  P.  C.,  5:5,  10:24 



Kennedy,  J.  W.,  4:24,  6:33,  6:34, 

12:23,  12:48 
Kingdon,  K.  H.,  11:6 
Kistiakowski,  G.  B.,  3:16,  12:23, 


Langmuir,  I.,  11:6 

Lauritsen,  C.  C.,  I 

Lawrence,  E.  O.,  3:15,  3:16,  3:17, 
5:7,  5:8,  5:17,  5:21,  5:31,  6:2, 
11:1,  11:2,  11:4,  11:5,  11:6, 
11:26,  11:29,  11:45 

Leverett,  M.  C.,  6:5,  7:15 

Lewis,  W.  K.,  3:16,  5:5,  6:6,  13:5 

Libby,  W.  F.,  10:2 

McBee,  E.  T.,  10:29 

Mack,  E.,  10:23 

MacKenzie,  K.  R.,  11:6 

McMillan,  E.,  6:34 

Manley,J.  H.,  6:39 

Marshall,  Gen.  G.  C.,  5:9,  5:21, 

Marshall,  Col.  J.  C.,  5:23,  5:28, 


Matthias,  Col.  F.  T.,  5:32 
Meitner,  L.,  1:53,  1:54 
Miles,  J.  B.,  7:50 
Miller,  W.  T.,  10:29 
Mills,  Adm.  E.  W.,  13:5 
Mitchell,  A.  C.  G.,  4:11 
Mohler,  F.  L.,  3:5 
Moore,  T.  W.,  6:5,  7:15 
Moses,  Gen.  R.  G.,  5:26 
Mulliken,  R.  S.,  3:16 
Murphree,  E.  V.,  5:4,  5:5,  5:8, 

5:17,  6:6,  6:8,  9:41,  10:2 
Murphy,  G.  M.,  1:21,  10:23 

Nichols,  Col.  K.  D.,  5:32,  10:34 
Nier,  A.  O.,  4:29,  10:1,  10:29,  11 :4 

Nix,  F.  C.,  10:26 
Norris,  E.  O.,  10:26 

Oliphant,  M.  L.  E.,  3:19,  11:6, 


Oppenheimer,  F.,  11:6 
Oppenheimer,  J.  R.,  5:31,  6:37, 

6:39,    11:6,    12:2,    12:5,   12:23, 

12:50,  A-6 

Parkins,  W.  E.,  11:6 
Parsons,  Capt.  W.  S.,  12:23 
Paxton,  H.  C.,  10:7,  10:26 
Pegram,  G.  B.,  1:53,  3:3,  3:4,  3:6, 

3:8,  3:9,  3:14,  3:20,  3:21,  3:22, 

4:2,  4:3,  4:5,  4:6,  4:46,  5:2,  5:4, 


Peierls,  R.,  10:4 
Perlman,  L,  6:33 
Peters,  B.,  11:6 
Purnell,  Adm.  W.  R.,  5:25,  5:26, 

5:27,  5:33 

Rayleigh,  Lord,  9:14 
Reynolds,  W.  B.,  11:6 
Richardson,  J.  R.,  11:6 
Roberts,  R.  B.,  3:5 
Rodden,  C.J.,  4:44,  6:16 
Roosevelt,   President  F.  D.,   3:4, 

3:5,  3:6,  3:7,  3:9,  3:22,  3:23, 

5:25,  5:33,  7:5 
Rosen,  R.,  10:29 
Ruhoff,  Col.,  6:19 
Rutherford,  E.,   1:6,   1:17,   1:38, 


Sachs,  A.,  3:4,  3:5,  3:6,  3:7 
Seaborg,  G.  T.,  4:24,  6:33,  6:34 
Segre,  E.,  4:24 
Serber,  R.,  6:39 
Simon,  F.,  5:14,  10:4 
Slack,  F.  G.,  10:2,  10:26 



Slade,  C.  B.,  10:7,  10:26 

Slater,  J.  C.,  3:15 

Slepian,J.,  11:2,11:6,  11:29 

Sloan,  D.  H.,  11:6 

Smith,  C.  S.,  12:23,  12:48 

Smith,  L.S.,  11:6,  11:29 

Smyth,  H.  D.,  3:14,  4:12,  4:27, 

4:28,  4:31,  5:2,  5:4,  5:18,  7:48, 

8:56,  11:1,  11:24,  11:29,  13:5 
Snell,  A-3 

Solberg,  Capt.  T.  A.,  13:5 
Somervell,  Gen.  B.,  5:25 
Spedding,  F.  H.,  6:5,  6:15,  6:17, 


Stearns,  J.  C.,  7:48 
Squires,  A.  M.,  9:32 
Stewart,  I.,  5:17 

Stimson,  H.  L.,  5:9,  5:21,  5:25,  7:5 
Stone,  Dr.  R.  S.,  6:5,  7:48,  8:70 
Strassmann,  F.,  1:53,  1:54 
Steyer,  Gen.  W.  D.,  5:21,  5:22, 

5:25,  5:27,  5:33 
Swearingen,  J.  S.,  10:28 
Szilard,  L.,  2:10,  2:11,  3:1,  3:4, 

3:5,  3:6,  3:8,  3:14,  4:2 

Taylor,  H.  S.,  9:40,  10:23,  10:26 
Teller,  E.,  3:1,  3:5,  3:9,  6:39 
Thiele,  E.  W.,  6:33 
Thomas,  C.  H.,  7:48 
Thomson,  G.  P.,  3:18 
Thomson,  J.  J.,  1 :35 
Thornton,  R.  L.,  11:6 
Tolman,  R.  C.,  5:31,  13:5 
Truman,  President  H.  S.,  5:34 
Turner,  L.  A.,  1:53,  1:58 
Tuve,  M.  A.,  3:8,  3:9,  4:29 

Urey,  H.  C.,  1:21,  3:3,  3:7,  3:8, 
3:9,  3:12,  3:14,  3:20,  3:21, 
3:22,  3:23,  4:32,  4:38,  4:46, 
5:2,  5:4,  5:7,  5:8,  5:12,  5:17, 
5:21,  5:31,  6:2,  6:33,  9:18, 
10:2,  10:23,  10:41 

VanVleck,J.  H.,  3:15 
Vernon,  H.  C.,  7:48,  8:56 

Wahl,  A.  C.,  4:24,  6:33,  6:34 
Wallace,  H.  A.,  3:22,  5:9,  5:21 
Walton,  E.  T.  S.,  1:38,  1:40 
Warren,  Col.  S.  L.,  5:32 
Watson,  Gen.  E.  M.,  see  Roosevelt, 

President  F.  D. 
Watson,  W.  W.,  7:48 
Watts,  G.  W.,  10:28 
Weil,  G.,  4:13,  8:27 
Weisskopf,  V.  F.,  3:1 
Weed,  L.  H.,  3:3 
Wensel,  H.  T.,  5:2,  5:4,  5:17 
Wever,  E.  G.,  3:3 
Wheeler,  J.  A.,  1:53,  1:57,  3:14, 

Whitaker,  M.  D.,  7:48,  7:50,  8:34 
Wigner,  E.,  3:1,  3:3,  3:4,  3:5,  3:6, 

3:8,  3:9,  4:12,  4:22,  4:27,  4:28, 

6:5,  6:33,  7:15,  7:48,  8:56 
Wilhelm,  H.  A.,  6:33 
Williams,  R.,  6:6,  7:50 
Wilson,  C.  T.  R.,  A-l 
Wilson,  R.  R.,  4:12,  11:24,  12:23 
Wilson,  V.  C.,  6:31 

Zinn,  W.  H.,  4:13,  6:31,  8:27,  8:61 


Actinium,  1:15,  1:16 

Advisory  Committee  on  Uranium, 
3:4,  3:5,  3:8,  3:9.  See  also  Urani- 
um Committee  (NDRC),  ND- 
RC  Section  S-l,  OSRD  Section 
S-l,  OSRD  Section  S-l  Execu- 
tive Committee 

Allis-Chalmers  Manufacturing 
Company,  11:19,  11:45 

Alpha  particle,  1:33,  2:32,  4:24; 
bombardment  by  1:17,  1:19, 
1:40;  product  of  nuclear  disin- 
tegration; 1:9,  1:15,  1:16,  1:38, 
1:51,  A-l 

Aluminum,  7:22 

Argonne  Laboratory,  8:27,  8:28, 

Atomic  number,  1:12,  1:15 

Atomic  structure,  1:6,  1:10,  1:11 

Bakelite  Corp.,  10:26 

Barium,  1:54 

Bell  Telephone  Laboratories,  Inc., 

Beryllium,  1:19,  1:40,  1:45,  2:10, 
2:13,  2:29,  2:36,  4:10,  4:14, 
4:18,  4:19,  4:43,  6:8,  6:22 

Beta  particles,  1:9,  1:15,  1:16, 
2:32,  4:24,  6:34,  8:17,  A-l 

Binding  energies,  nuclear,  l:31ff., 
1:36,  1:54 

Bomb,  atomic,  2:1,  2:3,  2:14, 
2:16,  2:23,  2:34,  2:35,  2:36,  3:5, 
4:25,  4:48,  4:49,  5:2,  5:10,  5:16, 
6:1,  6:32,  6:37,  11:28,  11:42, 
12:1,  13:1,  A-6 

Boron,    1:19,    1:40,    2:10,    4:42, 
6:11,  A-l,  A-3 

Cadmium,  3:11,  4:6,  7:27,  8:8,  A-4 
California  Institute  of  Technology, 

California,    University    of,     1:53, 

3:12,    4:7,    4:24,    4:25,    6:33, 

6:34,   6:38,    11:2,    11:6,    11:12, 

11:19,11:23,11:44,  11:45,  12:7, 

Canadian  Radium  and  Uranium 

Corp.,  6:10 
Carbide   and   Carbon   Chemicals 

Corp.,  10:2,  10:25,  10:11 
Carbon,  1:51,  2:10,  2:13,  3:5,  3:6, 

3:8,  4:2 
Carnegie  Institute  of  Washington, 

Centrifugal  isotope  separation, 

4:32,    4:37,    4:49,    5:6,    5:15, 

9:18,9:36,9:42,  11:16 
Chain  reaction,  1:50,  1:51,  1:56, 

2:3ff.,   2:23,   2:31,   2:32,    2:33, 

2:34,    2:36,    4:1,    4:2ff.,    4:47, 

5:7,  6:6,  6:21ff.,  8:3ff.,  12:8 
Chemical  analysis,  4:44,  7:34 
Chemical  exchange  method  of 

isotope  separation,   4:39,    9:24, 

Chemical  problems,  7:39,   11:35, 

Chicago,  University  of,  3:12,  4:7, 

4:11,    4:18,    4:20,    5:12,    6:27, 

6:33,6:38,7:8,7:9,8:50,  11:41, 




Chicago,  University  of,  12:32, 
13:1.  See  also  Metallurgical  Lab- 
oratory, 29 

Clinton  Engineer  Works,  7:8, 
10:33,  10:42,  11:44,  11:45,  13:1 

Clinton,  Tenn.,  7:1,  8:34,  6:44 

Cloud  chamber,  1:7,  12:35,  A-l 

Columbia  University,  1:53,  3:6, 
3:7,  3:10,  3:12,  4:2,  4:8,  4:9, 
4:13,  4:17,  4:22,  4:32,  4:38, 
4:39,  4:40,  4:41,  5:12,  8:56, 
10:1,  10:23,  10:26,  10:39,  11:41, 

Consolidated  Mining  and  Smelting 
Co.,  9:41 

Controls,  7:27,  8:8 

Cornell  University,  3:12,  6:28 

Cooling,  6:43,  7:13,  7:15,  7:16, 
7:18,  7:26,  8:8,  A-5 

Corrosion,  7:21,  7:25,  7:38,  10:17, 

Counter,  Geiger-Muller,  1:7,  4:6, 
12:35,  A-l 

Counter,  proportional,  A-l,  A-3 

Cross  Section,  nuclear,  1 :45ff., 
4:6,  12:12,  A-5.  See  also  nuclei 

Cyclotron,  1:40,  4:10 

Danger  coefficients,  8:28 

Decontamination,  8:23 

Detection  of  nuclear  particles,  4:6, 
4:13,  8:39,  8:67,  12:35,  A-l. 
See  also  Cloud  Chamber,  Coun- 
ter, Electroscope,  lonization 

Deuterium,  see  Heavy  water 

Deuteron,  1:21,  1:40,  6:34 

DSM  Project,  5:23,  7:4 

du  Pont,  E.  I.,  de  Nemours  and 
Co.,  Inc.,  6:13,  6:18,  6:40,  7:4, 
7:5,  7:6,  8:50,  9:40,  10:29, 
11:19,  7:50ff. 

Electrolytic     method     of    isotope 

separation,  9:25 

Electromagnetic  methods  of  iso- 
tope separation,  4:30,  5:7,  5:15, 
9:28,  ll:lff.  Calutron,  11:2, 
11:10,  11:20.  Ion  source,  9:30, 
11:4,  11:5,  11:25.  Isotron,  9:31, 
11:2,  11:24.  Magnetron  (ion 
centrifuge),  9:31,  11:2,  11:29. 
Space  charge  effects  9:30,  11:5, 

Electron,  1:11,  1:12,  1:23 
Electroscope,  1:7,  12:35,  A-l 
Energy,  conservation  of,  1:2 
Enrichment  factor,  9:8,  9:14,  9:20, 

9:30,  10:6,  10:7,  11:14,  11:21 
Equilibrium,  radioactive,  8:18 

Fast  fission  effect,  8:9 

Feasibility  Report,  6:6,  6:39,  6:43, 

Fission,  1:3,  1:53,  1:55,  1:57,  2:3, 
2:14,  2:19,  2:21,  2:35,  6:38, 
12:16.  See  also  Neutron,  Plu- 
tonium, Uranium 

Fission  Products,  1:53,  1:56,  1:57, 
2:10,  2:32,  4:6,  7:29,  7:35,  8:17, 

Fractional  distillation  method  of 
isotope  separation,  9:16,  9:38 

Gamma  rays,  1:9,  1:15,  1:19,  2:32, 
8:5,  8:17,  A-l 

Gaseous  diffusion  method  of  iso- 
tope separation,  4:32,  4:37, 
4:47,  4:49,  5:6,  5:12,  5:14, 
9:14, 10:lff.,  10:14, 10:26, 11:16, 

General  Electric  Co.,  11:19,  11:45 

George  Washington  University, 

Graphite,  2:9,  2:19,  2:30,  2:31, 
2:36,  3:5,  3:7,  3:11,  4:2,  4:4, 



Graphite,   4:6,   4:13,   4:42,   4:47, 
5:14,  6:8,  6:28,  6:29 

Half  Life,  1:16,  6:28,  8:17.  See  also 
Nuclei  involved 

Hanford  Engineer  Works,  7:8, 
7:12,  8:50,  10:78,  13:1 

Harshaw  Chemical  Co.,  6:13, 

Health  hazards,  2:32,  2:36,  4:26, 
6:5,  7:16,  7:19,  7:26,  7:28,  7:35, 
7:36,  7:41,  8:35,  8:54,  8:63, 
8:67,  8:78,  A-5.  See  also  Radio- 
active poisons 

Heavy  water,  3:14,  5:6,  5:12, 
5:14.  As  moderator,  2:10,  2:13, 
2:28,  4:38,  4:47,  5:14,  6:8, 
6:43,  6:45,  8:32,  8:55.  Produc- 
tion, 4:39,  9:36 

Helium,  see  Alpha  particle  cooling 

Hooker  Electrochemical  Co.,  10:29 

Houdaille-Hershey  Corp.,  10:26 

Indiana,  University  of,  4:11,  6:38 

Indium,  4:6 

Iodine,  4:6 

Ion    accelerator,     1:40.    See    also 

lonization     chamber,     1:7,     4:6, 

12:35,  A-l 
Iowa   State   College,    3:12,   6:16, 

6:33,  A-4 
Isobars,  1:14 
Isotopes,    1:14,    1:29.    Separation 

of,  2:12,  5:7,  5:15,  6:2,  9:lff., 


J.  A.  Jones  Construction  Co.,  Inc., 
10:24,  10:41 

John  and  Mary  Markel  Founda- 
tion, 11:12 

Johns  Hopkins  University,  1:53, 

Joint  New  Weapons  Committee, 

Kaiser  Wilhelm  Institute,  3:6 
Kellex  Corp.,   9:32,   10:2,   10:24, 
10:26,  10:41 

Lattice,  2:11,  2:19,  2:31,  4:4,  4:6, 
4:8,  4:13,  4:15,  4:16,  4:20,  4:22, 
4:23,  4:27,  5:15,  6:21,  6:22, 
6:28,  6:42,  6:43,  7:1,  7:3,  7:18, 
7:19,  7:46,  8:4,  8:9,  8:21,  8:28, 
8:36,  8:59,  A-4.  Poisoning  of, 

Lithium,  1:19,  1:38,  1:48,  2:10, 

Los  Alamos,  N.  M.,  7:48,  12:4, 
12:18,  13:1,  A-6 

Mallinckrodt     Chemical     Works, 

6:12,  6:18,  6:19,  6:22 
Manhattan    District,    5:23,    5:29, 

5:30,    5:32,    6:18,    7:4,    10:23, 

11:11,  11:39,  12:2,  12:7,  13:1 
Mass,  conservation  of,  1:2,  A-l 
Mass-Energy  equivalence,  1 :4, 1 :6, 

1:8,  1:38 

Mass  number,  1:13,  1:15,  1:35 
Mass  spectrograph,  1:35,  9:28ff. 
Metal  Hydrides  Co.,  4:41,  6:10, 

6:15,  A-4 
Metallurgical   Laboratory,   6:2, 

6:33,  6:35,  7:1,  7:9,  7:10,  7:40, 

7:46,    7:50,    8:2,    8:22,    8:34, 

8:56,  11:2,  11:19,  12:2,  12:32. 

See  also  Chicago,  University  of 
Military  Policy  Committee,  5:25, 

Minnesota,    University    of,    3:12, 

Moderator,     2:8ff.,     2:11,     2:13, 

2:14,   2:19,   2:20,   2:28ff.,    4:1, 



Moderator,   4:8,    8:8,    12:40.    See 

also  Beryllium,  Graphite,  Heavy 

Multiplication  factor,  4:13,  4:15, 

4:16,    6:10,    6:11,    6:22,    7:19, 

8:12,  8:15,  A-4 
M.  W.  Kellogg  Co.,  10:2,  10:23, 

10:32,    10:11.    See   also    Kellex 


National  Bureau  of  Standards, 
6:16,  6:20,  6:38,  9:31 

National  Carbon  Co.,  Inc.,  6:20, 
10:23,  A-4 

NDRC,  2:37,  3:4,  3:9,  3:10. 
Section  S-l,  3:14,  4:14,  4:31, 
5:2.  See  also  Advisory  Commit- 
tee on  Uranium 

Naval  Research  Laboratory,  4:36, 
11:38,  11:39,  11:47 

Navy  Department,  2:1,  3:4,  3:6, 
3:11,  3:12,  4:36,  10:2,  11:39, 

Neptunium,  1:58,  2:19,  6:34,  8:18 

Neutron,  l:18ff.,  l:23ff.,  1:33, 
Absorption  of,  1:47,  2:13,  1:57, 
2:3,  2:8ff.,  2:12,  2:19,  8:4,  8:6, 
12:8,  4:8,  4:19,  6:20ff.,  A-5. 
Delayed,  6:23,  A-3,  A-4.  Detec- 
tion, see  Detection  of  nuclear 
particles.  Fast,  1:57,  2:1,  2:21, 
2:14,  2:10,  2:12,  4:25,  6:37,  8:9. 
Fission  induced  by,  1:52,  1:57, 
2:21,  3:6,  4:6,  4:24,  4:25,  10:1. 
Resonance,  2:12,  4:6,  4:8. 
Sources,  1:40,  4:10,  4:14,  8:31, 
8:32,  12:38,  A-5.  Thermal, 
1:57,  2:1,  2:9,  2:21,  2:14,  3:6, 
4:6,  4:25,  8:31,  8:39,  10:1, 
12:40,  A-l 

Nitrogen,  1:17,1:51 

Nuclear  disintegrations,  1:15, 1:20, 
1:38,  1:49.  Artificial,  1:17 

Nuclear  Reaction,  l:38ff.  Nota- 
tion for  1:43.  See  also  Fission, 
Nuclear  disintegration,  Nuclei 

Nucleus,  1:11,  1:12.  Structure  of, 

Oak  Ridge,  Tenn.,  see  Clinton 
Engineer  Works 

OSRD,  5:2,  5:9,  10:2,  10:23, 
11:26,  12:2.  Section  S-l,  5:3, 
5:17,  5:28,  7:1,  10:33,  11:4, 
11:18.  See  also  Advisory  Com- 
mittee on  Uranium 

Oxygen,  1:17,  1:51 

Philadelphia  Navy  Yard,  11:39, 

Pile,  2:20.  See  also  Argonne  Labo- 
ratory, Clinton  Engineer  Works, 
Hanford  Engineer  Works,  Lat- 
tice, West  Stands  pile 

Planning  Board,  5:5,  5:6,  6:8 

"Pluto,"  8:68 

Plutonium,  1:58,  2:23,  2:36,  3:17, 
4:24,  4:25,  5:7,  5:21,  6:1,  6:4, 
6:34,  6:35,  8:lff.,  12:10.  Fission, 
1:58,  2:14,  4:24,  4:48.  Produc- 
tion, 2:19,  5:14,  6:32,  6:41,  7:3, 
7:8ff.,  7:18ff.,  8:lff.,  9:44, 11:32, 
2:18.  Radioactivity,  1:58,  4:24 

Poison  gas,  4:27 

Polonium,  1:19 

Positron,  1:21,  1:51 

Power,  1:48,  2:3,  2:16,  2:34,  3:5, 
3:14,  4:48,  6:29,  7:26,  13:5 

Princeton  University,  3:12,  4:7, 
4:9,  4:11,  4:12,  4:18,  4:22,  5:12, 
9:40,  11:2,  11:44 

Protoactinium,  1:57,  2:21 



Proton,  1:13,  1:17,  1:19,  1:23, 
1:24,  1:33,  1:38,  1:40,  1:48, 
1:51,  4:10,  A-l 

Purdue  University,  6:38 

Radioactivity,  1:6,  1:7,  1:15,  2:32. 
Artificial,  1:26,  1:30,  4:6,  4:26, 
A-l.  Elements  showing,  1:16. 
Measurement  of,  see  Cloud 
chamber,  counter,  Detection  of 
nuclear  particle,  Electroscope, 
lonization  chamber.  Poisonous 
effects,  4:26,  4:48;  see  also 
Health.  Products  of,  see  Alpha 
particles,  Beta  particles,  Gam- 
ma rays,  Nuclear  disintegra- 
tion, Radioactive  series 

Radium,  1:15,  1:16,  2:24,  2:32 

Reference  Committee  (National 
Research  Council),  3:3 

Reflector  (tamper),  12:13,  12:24, 

Relativity,  1:4 

Relaxation  time,  A-4 

Reproduction  factor,  see  Multipli- 
cation factor 

Research  Corporation,  1 1 :4 

Resonance  escape  probability,  8:10 

Reviewing  Committee  (National 
Academy  of  Sciences)  3:15, 
3:16,  3:18,  3:21,  3:23,  4:23, 
4:24,  4:27,  4:48,  6:6,  6:39,  7:1 

Rhodium,  4:6 

Rice  Institute,  6:38 

Richland,  Wash.,  see  Hanford  En- 
gineer Works 

Rockefeller  Foundation,  11:12 

Scintillations,  A-l 
Shielding,  7:28,  8:67 
"Sneezy,"  8:68 
Speer  Carbon  Co.,  6:20,  A-4 

Standard   Oil   Development   Co., 

3:12,  5:12,  9:41,  9:42 
Stanford  University,  6:38 
Stone   and   Webster   Engineering 

Corp.,  10:34,  11:19,  11:45 

Tamper,  see  Reflector 

Tennessee  Eastman  Corp.,  11:19, 

11:34,  11:45 

Thermal  diffusion  method  of  iso- 
tope separation,  4:36,  9:21, 

Thermal  utilization  factor,  8:11 
Thorium,  1:15,  1:16,  1:57,  2:21, 

2:24,  2:25 

Tolman  Committee,  13:5,  13:6 
Top  Policy  Group,  3:22,  5:9 
Transuranic  elements,  4:25 

Union  Miniere,  3:7 

Union  Carbide  and  Carbon  Corp., 

United  States  Graphite  Co.,  4:42 

Units,  A-l 

Uranium,  1:7,  1:15,  2:23,  2:24, 
2:26,  2:27,  6:4,  6:34,  11:36. 
Fission,  1:3,  1:52,  1:57,  2:1,  2:3, 
2:10,  2:12,  2:13,  2:14,  2:32,  3:6, 
8:8ff.,  2:35,  2:36,  4:49,  8:5, 
10:1,  8:9,  3:16.  Isotopes,  1:16, 
2:18,  2:4,  4:34,  4:47,  5:21,  6:1, 
10:1,  4:24.  Isotope  separation, 
see  Isotope  separation.  Neutron 
absorption,  2:3,  2:6,  2:11,  2:19, 
4:2,  4:6,  4:7,  4:24,  4:25,  8:5. 
Uranium  production,  4:41,  6:10, 

Uranium  Bromide,  1 1 :4 

Uranium  Committee  (NDRC), 
3:9,  3:14,  4:36,  9:30,  11:26, 
11:29.  See  also  Advisory  Com- 
mittee on  Uranium 

264  Index 

Uranium  Hexafluoride,  4:33,  4:36,      Washington  University  (St.  Louis), 
9:14,  10:3,  10:11  6:34 

Westinghouse  Electric  and  Manu- 
Virginia,  University  of,  3:12,  5:12,          facturing  Co.}  6:10,  6:13,  6:14, 

9:42  9:42,  11:19,  11:45,  A-4 

War  Department,  7:5,  3:6,  3:12,      West  Stands  pile,  8:27,  A-4 
5:9,  11:18  Wisconsin,  University  of,  6:38